JBIDE-16843 - Restore /documentation/whatsnew (drop the '_') and get individual pages generated

Fixed missing pages on sidenavbar
Reduced number of versions labelled with "Stable" / "Development" / "Nightly", shared code
between "Downloads" and "Whatsnew" parts.
Added multiple versions on /documentation/whatsnew
Moved remaining article from /documentation/_whatsnew to /documentation/whatsnew