No more whitespace checks in non-.hs files

[ulysses4ever]

When switching to fourmolu for formatting the codebase we killed the fix-whitespace workflow, which, I believe, checked not only .hs-files but also .md, .rst and maybe something else. Can we get this functionality back? I think it was useful. Maybe we could be smart about running it in GitHub actions only when any non-.hs touched (which will save some time on good bunch of runs).

/cc @Kleidukos