Update versions on website, simplify access to release notes and schedules

The main page of the Apertis website was pointing at release schedules
of releases that had already been released and not pointing at the
currently released versions of Apertis. Whilst remedying this it became
apparent that we are storing the "current" versions in 3 different places
thus increasing the likelyhood that one will be missed on any update.

So:
- Merge the "release notes" and "release schedules" pages into a unified
  release page
- Point to this unified page and the current releases on the main page
- Update links as appropriate
- Add missing release schedules for next releases (with TBD dates for now)

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>