Release notes for 5.4.6.

Installation instructions for the stable release are generic, at https://doc.ez.no/display/EZP/5.4.x+Update+Instructions. Specifics of 5.4.6 will be added there. But if we update that page during the RC, it will show instructions for a release that isn't public yet (the release notes are private).

This has so far been handled by putting the update instructions for 5.4.6 in the release notes during the RC, and copy/pasting them when the release notes are made public.