[MSITE-911] site:attach-descriptor should also attach resources

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MSITE-911](https://issues.apache.org/jira/browse/MSITE-911?redirect=false)** and commented

Any additional resources put into `src/site/resources` (according to https://maven.apache.org/guides/mini/guide-site.html#adding-extra-resources) should be also included as attached artifact and extracted when generating a site.
Otherwise it is impossible to e.g. globally define a logo or CSS referenced locally.

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**Issue Links:**
- [DOXIASITETOOLS-359](https://issues.apache.org/jira/browse/DOXIASITETOOLS-359) Optionally allow files in one source directory overwrite files from another
(_**"is blocked by"**_)

**Remote Links:**
- [GitHub Pull Request #216
](https://github.com/apache/maven-site-plugin/pull/216)
- [Maven Parent Issue #216
](https://github.com/apache/maven-parent/issues/216)