Update doc generation script for Java 11+
The org.eclipse.etrice.doc generation script doesn't work anymore with Java 11+, due to Stagen having a deprecated dependency on javax.annotation. The dependency needs to be downloaded separately and enabled as a module.
Also make sure the documentation for the documentation process is up to date (some updates have already been made to existing readme in merge request), and make sure that it is discoverable (linked from top-level readme/developer wiki page?)