[MJAVADOC-697] Allowing to register alternative Javadoc implementation

**[Toni Epple](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=dukescript)** opened **[MJAVADOC-697](https://issues.apache.org/jira/browse/MJAVADOC-697?redirect=false)** and commented

The Maven compiler plugin allows [different implementations of compilers](https://maven.apache.org/plugins/maven-compiler-plugin/non-javac-compilers.html) to plug in. One of the very useful ones is the [retrofit compiler for Java](http://frgaal.org/) that allows one to use syntax of JDK-16 while running on old JDKs.

The compiler works fine, however the problem comes when one wants to generate Javadoc. Because the source code is using newer language constructs, classical `javadoc` from old JDKs fails. One either has to generate the Javadoc on a newest JDK (which beats the purpose of retrofit compiler) or give up on Javadoc. Or...

Let's enhance the Maven Javadoc Plugin to support alternative implementations of Javadoc, just like the Maven Compiler Plugin does with compilers!

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**Affects:** 3.1.1

**Remote Links:**
- [GitHub Pull Request #106
](https://github.com/apache/maven-javadoc-plugin/pull/106)

Start with the Maven Javadoc Plugin and compare its configuration and implementation with the Maven Compiler Plugin's support for alternative compilers, as described in the linked documentation. Done means an alternative Javadoc implementation can be registered and used to generate documentation for sources requiring newer Java syntax.

Written by the indexing model from the issue text.