`cabal check` should warn when readme and other dev docs file exist in the working tree, but are not included in `extra-doc-files`

Offspring of #3964.

**Describe the feature request**
Browsing Hackage or flora.pm would be a much nicer experience if everyone added their devel docs to `extra-doc-files`.
Sometimes this is not the case, because people forget or because they choose not to (as rendering on Hackage might be off).

It would be nice if `cabal check` looked for all such files in the working tree and hinted the author to add them.
As now `cabal check` warns about *changelogs* but not other files (e.g. `README`s).

The check would have to be granular, so users can `--ignore` specific warnings.