Build Lucene Javadocs as a standalone artifact for Solr users

Solr relies on a lot of Lucene packages. Some of the classes (e.g. Tokenizers) have a lot of documentation in the Javadoc.

However, the JavaDoc shipped with Solr only covers Solr specific classes. And full Lucene download is rather large (50M) to get them just for JavaDocs.

It would be useful to have a separate download of just Lucene Javadocs for those people who want/need to work offline and want to have all the relevant documentation.