gh-101100: Test only Doc/ files in nit-picky mode

[hugovk]

Follow on from #102513.

To fix #102513 (comment).

The problem is that the https://github.com/Ana06/get-changed-files action gives an error if there's a filename with a space, for example, a news file such as Misc/NEWS.d/next/Core and Builtins/2023-03-17-12-09-45.gh-issue-100982.Pf_BI6.rst.

The error suggests using another format:

Error: One of your files includes a space. Consider using a different output format or removing spaces from your filenames. Please submit an issue on this action's GitHub repo.

We can get in CSV or JSON format instead:

• https://github.com/Ana06/get-changed-files#get-all-added-and-modified-files-as-csv

But that would require some refactoring, as we need to touch the list of files.

The quick fix is to filter for files in the Doc/ directory, to only consider those:

• https://github.com/Ana06/get-changed-files#filtering

It would of course be good to check the NEWS files, but it's less urgent than fixing the CI and unblocking other PRs!

Demo

hugovk#45 is a PR that modifies a Doc/ file plus a Misc/NEWS.d/next/Core and Builtins/ file, and the "Docs / Docs" CI check fails, as expected.

hugovk#44 is the same but also has this change, and "Docs / Docs" passes.

• Issue: Fix all Sphinx reference warnings in the documentation #101100

[AlexWaygood]

Makes sense as a short-term fix!

[CAM-Gerlach]

LGTM, thanks @hugovk for the quick fix!

[encukou]

We can get in CSV or JSON format instead

IMO, the best fix would be to avoid spaces in CPython sources, so we can use simpler (though less generally correct) tooling.
Here's the blurb issue for it: python/core-workflow#186