Merge pull request #390 from feketegy/feature/docs-revamp

Rearranged sidebar menu
Rewrote menu from scratch, added new pages (refs #4723)
Added new elements to menu (refs #4723)
Added contributing, conventions & style pages (refs #4723)
Changelog, about page (refs #4723)
Installation page (refs #4723)
Cleanup (refs #4723)
Added a i18n and messages sections (refs #4723)
Added CFS page (refs #4723)
Added request flow page (refs #4723)
Moved routing page (refs #4723)
Moved this subsection into a separate page (refs #4723)
Navigation changes (refs #4723)