Java source code license header check should skip docs directory

As of now, after building docs page via Jekyll, Maven invocation fails due to the license header check.

```
suztomo@suxtomo24:~/cloud-opensource-java$ mvn clean install -Dmaven.test.skip
[INFO] Scanning for projects...
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Build Order:
[INFO]
[INFO] Google Cloud Platform Supported Libraries [pom]
[INFO] BOM Parent [pom]
[INFO] Cloud Platform Supported Libraries Upper Bounds Check [pom]
[INFO] Cloud Tools Open Source Code Hygiene Tooling [pom]
[INFO] Maven Dependency Graph Analyzer [jar]
[INFO] integration-tests [jar]
[INFO] Cloud Tools Open Source Code Hygiene Dashboard [jar]
[INFO] Linkage Checker Enforcer Rule [jar]
[INFO] Linkage Monitor [jar]
[INFO]
[INFO] -------------------< com.google.cloud:libraries-bom >-------------------
[INFO] Building Google Cloud Platform Supported Libraries 4.1.2-SNAPSHOT [1/9]
[INFO] --------------------------------[ pom ]---------------------------------
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ libraries-bom ---
[INFO]
[INFO] --- maven-install-plugin:2.4:install (default-install) @ libraries-bom ---
[INFO] Installing /usr/local/google/home/suztomo/cloud-opensource-java/boms/cloud-oss-bom/pom.xml to /usr/local/google/home/suztomo/.m2/repository/com/google/cloud/libraries-bom/4.1.2-SNAPSHOT/libraries-bom-4.1.2-SNAPSHOT.pom
[INFO]
[INFO] ---< com.google.cloud.tools.opensource:cloud-tools-opensource-boms >----
[INFO] Building BOM Parent 1.0-SNAPSHOT [2/9]
[INFO] --------------------------------[ pom ]---------------------------------
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ cloud-tools-opensource-boms ---
[INFO]
[INFO] --- maven-install-plugin:2.4:install (default-install) @ cloud-tools-opensource-boms ---
[INFO] Installing /usr/local/google/home/suztomo/cloud-opensource-java/boms/pom.xml to /usr/local/google/home/suztomo/.m2/repository/com/google/cloud/tools/opensource/cloud-tools-opensource-boms/1.0-SNAPSHOT/cloud-tools-opensource-boms-1.0-SNAPSHOT.pom
[INFO]
[INFO] --------< com.google.cloud.tools.opensource:upper-bounds-check >--------
[INFO] Building Cloud Platform Supported Libraries Upper Bounds Check 2.0.0-SNAPSHOT [3/9]
[INFO] --------------------------------[ pom ]---------------------------------
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ upper-bounds-check ---
[INFO]
[INFO] --- maven-enforcer-plugin:3.0.0-M3:enforce (enforce) @ upper-bounds-check ---
[INFO]
[INFO] --- maven-install-plugin:2.4:install (default-install) @ upper-bounds-check ---
[INFO] Installing /usr/local/google/home/suztomo/cloud-opensource-java/boms/upper-bounds-check/pom.xml to /usr/local/google/home/suztomo/.m2/repository/com/google/cloud/tools/opensource/upper-bounds-check/2.0.0-SNAPSHOT/upper-bounds-check-2.0.0-SNAPSHOT.pom
[INFO]
[INFO] -------------< com.google.cloud.tools:dependencies-parent >-------------
[INFO] Building Cloud Tools Open Source Code Hygiene Tooling 1.1.4-SNAPSHOT [4/9]
[INFO] --------------------------------[ pom ]---------------------------------
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ dependencies-parent ---
[INFO]
[INFO] --- license-maven-plugin:3.0:check (default) @ dependencies-parent ---
[INFO] Checking licenses...
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/concurrent-ruby-1.1.5/ext/concurrent-ruby/com/concurrent_ruby/ext/SynchronizationLibrary.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/concurrent-ruby-1.1.5/ext/concurrent-ruby/com/concurrent_ruby/ext/jsr166e/LongAdder.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/concurrent-ruby-1.1.5/ext/concurrent-ruby/com/concurrent_ruby/ext/jsr166e/Striped64.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/concurrent-ruby-1.1.5/ext/concurrent-ruby/com/concurrent_ruby/ext/AtomicReferenceLibrary.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/concurrent-ruby-1.1.5/ext/concurrent-ruby/com/concurrent_ruby/ext/JavaAtomicFixnumLibrary.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/concurrent-ruby-1.1.5/ext/concurrent-ruby/ConcurrentRubyService.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/concurrent-ruby-1.1.5/ext/concurrent-ruby/com/concurrent_ruby/ext/jsr166e/nounsafe/Striped64.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/concurrent-ruby-1.1.5/ext/concurrent-ruby/com/concurrent_ruby/ext/JavaAtomicBooleanLibrary.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/test/http_parser/lolevel/TestNoOverflowLongBody.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/concurrent-ruby-1.1.5/ext/concurrent-ruby/com/concurrent_ruby/ext/jsr166e/ConcurrentHashMapV8.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/test/http_parser/lolevel/Responses.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/test/http_parser/lolevel/Util.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/test/http_parser/lolevel/Test.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/test/http_parser/lolevel/ParseUrl.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/test/http_parser/lolevel/WrongContentLength.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/org/ruby_http_parser/RubyHttpParser.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/concurrent-ruby-1.1.5/ext/concurrent-ruby/com/concurrent_ruby/ext/jsr166e/nounsafe/ConcurrentHashMapV8.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/concurrent-ruby-1.1.5/ext/concurrent-ruby/com/concurrent_ruby/ext/JRubyMapBackendLibrary.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/concurrent-ruby-1.1.5/ext/concurrent-ruby/com/concurrent_ruby/ext/JavaSemaphoreLibrary.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/concurrent-ruby-1.1.5/ext/concurrent-ruby/com/concurrent_ruby/ext/jsr166y/ThreadLocalRandom.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/concurrent-ruby-1.1.5/ext/concurrent-ruby/com/concurrent_ruby/ext/jsr166e/ConcurrentHashMap.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/concurrent-ruby-1.1.5/ext/concurrent-ruby/com/concurrent_ruby/ext/jsr166e/nounsafe/LongAdder.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/impl/http_parser/lolevel/HTTPDataCallback.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/impl/http_parser/HTTPMethod.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/impl/http_parser/HTTPErrorCallback.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/impl/http_parser/FieldData.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/impl/http_parser/HTTPException.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/impl/http_parser/ParserType.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/impl/http_parser/Util.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/impl/http_parser/HTTPCallback.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/impl/http_parser/HTTPParser.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/impl/http_parser/HTTPDataCallback.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/test/http_parser/lolevel/Upgrade.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/test/http_parser/lolevel/UnitTest.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/test/http_parser/lolevel/TestLoaderNG.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/test/http_parser/lolevel/TestHeaderOverflowError.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/test/http_parser/lolevel/Url.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/test/http_parser/lolevel/Message.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/test/http_parser/lolevel/Requests.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/thread_safe-0.3.6/ext/org/jruby/ext/thread_safe/jsr166e/LongAdder.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/thread_safe-0.3.6/ext/org/jruby/ext/thread_safe/jsr166e/ConcurrentHashMap.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/thread_safe-0.3.6/ext/org/jruby/ext/thread_safe/jsr166e/nounsafe/ConcurrentHashMapV8.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/thread_safe-0.3.6/ext/org/jruby/ext/thread_safe/jsr166e/Striped64.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/thread_safe-0.3.6/ext/org/jruby/ext/thread_safe/jsr166e/nounsafe/LongAdder.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/thread_safe-0.3.6/ext/org/jruby/ext/thread_safe/jsr166e/nounsafe/Striped64.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/thread_safe-0.3.6/ext/org/jruby/ext/thread_safe/jsr166y/ThreadLocalRandom.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/thread_safe-0.3.6/ext/org/jruby/ext/thread_safe/JRubyCacheBackendLibrary.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/thread_safe-0.3.6/ext/thread_safe/JrubyCacheBackendService.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/thread_safe-0.3.6/spec/src/thread_safe/SecurityManager.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/RubyHttpParserService.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/impl/http_parser/ParserSettings.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/impl/http_parser/HTTPParserUrl.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/impl/http_parser/lolevel/ParserSettings.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/impl/http_parser/lolevel/HTTPErrorCallback.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/impl/http_parser/lolevel/HTTPCallback.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/http_parser.rb-0.6.0/ext/ruby_http_parser/vendor/http-parser-java/src/impl/http_parser/lolevel/HTTPParser.java
[WARNING] Missing header in: /usr/local/google/home/suztomo/cloud-opensource-java/docs/vendor/bundle/ruby/2.5.0/gems/thread_safe-0.3.6/ext/org/jruby/ext/thread_safe/jsr166e/ConcurrentHashMapV8.java
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO]
[INFO] Google Cloud Platform Supported Libraries 4.1.2-SNAPSHOT SUCCESS [ 0.152 s]
[INFO] BOM Parent 1.0-SNAPSHOT ............................ SUCCESS [ 0.005 s]
[INFO] Cloud Platform Supported Libraries Upper Bounds Check 2.0.0-SNAPSHOT SUCCESS [ 0.330 s]
[INFO] Cloud Tools Open Source Code Hygiene Tooling 1.1.4-SNAPSHOT FAILURE [ 1.841 s]
[INFO] Maven Dependency Graph Analyzer 1.1.4-SNAPSHOT ..... SKIPPED
[INFO] integration-tests 1.2.0-SNAPSHOT ................... SKIPPED
[INFO] Cloud Tools Open Source Code Hygiene Dashboard 1.1.4-SNAPSHOT SKIPPED
[INFO] Linkage Checker Enforcer Rule 1.1.4-SNAPSHOT ....... SKIPPED
[INFO] Linkage Monitor 1.1.4-SNAPSHOT ..................... SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 2.525 s
[INFO] Finished at: 2020-02-26T15:07:06-05:00
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal com.mycila:license-maven-plugin:3.0:check (default) on project dependencies-parent: Some files do not have the expected license header -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException
[ERROR]
[ERROR] After correcting the problems, you can resume the build with the command
[ERROR] mvn -rf :dependencies-parent

```

從由 `mvn clean install -Dmaven.test.skip` 呼叫的 Maven license-maven-plugin 檢查開始，並檢查儲存庫的原始碼路徑是如何選取的。將產生的 `docs` 目錄從 Java 標頭檢查中排除，然後在建置 Jekyll 文件後重新執行 Maven 命令，以確認授權檢查不再回報 `docs/vendor` 下的檔案。

由索引模型根據 Issue 內容生成。