Help users restore a missing or deleted branch from MR

This is a follow-up to UI text update for source/target branch does no... (#346384 - closed). Related, we have Undo removing a branch (#18633) to add a way to restore deleted branches through the UI.

Problem

When a source or target branch goes missing/is deleted we show this message in the MR:

It would be great if we helped users “restore” the branch. We only mention that option, but no further explanation is given on “how to restore missing/deleted branches”. Some of the top search results:

1. https://stackoverflow.com/questions/69761824/can-a-deleted-gitlab-branch-be-restored
2. https://forum.gitlab.com/t/accidentally-deleted-branch-how-to-restore/37386
3. https://riptutorial.com/git/example/2446/recover-a-deleted-branch

Also, we should not encourage users to merge manually via CLI.

Proposal

1. Add instructions in the documentation on how to restore a missing/deleted branch.
2. Update that message to point to the new documentation.