Document how to check if the cabal file format is changed by a PR or a release and if the change is guarded by a proper format version check

[Mikolaj]

The goal is to have clear easy instructions in the PR review checklist and a new point in the major-forcing release checklist. It should both describe how to easily determine if file format is modified and how to correctly guard the modification behind a version (and how to create a new version if not yet present).

I wonder if really the best way to check it is by inspecting code, but if so, which files exactly? Is it possible to get a hint by verifying if the hash checksums that are in the code got changed? I couldn't find anything about the hash checksums in the cabal manual nor on the wiki. Is there anything about the file format in our docs? I see the file format is documented in https://cabal.readthedocs.io/en/stable/cabal-projectindex.html and this is auto-generated, so we could reverse-engineer something from that.