changelogs are docs (backport #10354)

[mergify]

We don't mark changelogs as documentation, so CI unnecessarily does full checks when we add changelogs. Correct this.

NOTE: we only accept changelog files from top-level subdirectories. There are changelog files in package tests that must be considered "code" from the standpoint of CI.

Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).

---

This is an automatic backport of pull request #10354 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 871c0d4 has failed:

On branch mergify/bp/3.12/pr-10354
Your branch is up to date with 'origin/3.12'.

You are currently cherry-picking commit 871c0d46a.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   .github/workflows/bootstrap.skip.yml
	modified:   .github/workflows/bootstrap.yml
	modified:   .github/workflows/validate.skip.yml

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   .github/workflows/check-sdist.yml
	both modified:   .github/workflows/validate.yml

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally