Review and revise plugin documentation

Changes made in the plugin documentation:

• Javadocs
• APTs: index, usage, examples
• required tags for docck in the plugin's pom.xml
• FAQ page

The plugin docs are now ready for review

• goal links incorrect on front page
• usage indenting seems a bit more than usual 2 spaces
• use "mvn site" instead of site:site, and don't refer to "m2" on usage guide
• on usage guide, you do not need to put the jxr plugin into the pom at all to generate from the command line (and doing so from reporting is sufficient and recommended). I'd remove the pom sample and just leave the commands.
• remove FAQ/comment out from nav (the only question is answered on the front page already
• remove the output directories for javadoc in the pom example (hardcoded paths are a bad example to set). I'd just refer to needing to specify the javadoc plugin in the reporting section, and that if you customise the destdir then you need to customise the javadocdir in jxr
• suggest removing the example xref and just linking to the actual one for the plugin.

thanks!

Revised the docs and applied the comments from above. Thanks

looks good to me