Add the initial release doc for the 3.0 release

And update the upgrade check to ensure users are on the latest chart minor version before upgrading.

This aligns the chart version check practice with the same upgrade method we use for GitLab versions, and is necessary for the 3.0 major release, as several of our dependencies like prometheus and cert-manager can be upgraded smoothly by stepping the upgrades, so we need to be upgraded to their 2.6 versions before being upgraded any further.

Part of #1794 (closed)