`cabal-version` semantics

**What is wrong with the docs?**
I don't see a place where the semantics of `cabal-version` is documented. It could go to `CONTRIBUTING.md` (as a start?) because Cabal devs contribute features that prompt version bumps. Another place is the log for the version updates in the manual:

- https://cabal.readthedocs.io/en/3.10/file-format-changelog.html

**Additional context**
We have been getting requests to clarify the policy around `cabal-version` more often lately. E.g.

- https://github.com/haskell/cabal/pull/9535#issuecomment-1866354715

One place that looks relevant is https://github.com/haskell/cabal/wiki/Package-Description-Semantics but it looks completely in the woods and ancient.

A recent and very relevant discussion:

- https://github.com/haskell/cabal/issues/9331