docs: confluence-publish's wiki info lives in a plugin now.

also, give a sample ConfluencePublish goal.

Bugs closed: 723

Reviewed at https://rbcommons.com/s/twitter/r/1299/
Sync of https://github.com/pantsbuild/pants/commit/c605f305d23340aa84b6d63bd624a5cb4bcbf0c1