CI: verify docs are up to date

[pajod]

New CI task: complain if docs/source/settings.rst has changed after invoking sphinx-build, indicating someone (me) has forgotten to run make -C docs html

• Its not a tox env because I found it easier to ask git whether the file has changed.
• It will also fail if sphinx-build fails. This is good. We should know when that happens (note however, that this does not yet fail on a mistyped reference. that is, so far, just a warning.)

Proof that this works can be found in the earlier CI run of this very PR:

• https://github.com/benoitc/gunicorn/actions/runs/9185099713/job/25258482749
• it fails - which is to be expected, because only a later commit fixes includes the updated rst file.

Assumption:

• no need to duplicate between news.rst / 2023-news.rst - just move the content to referenced-in-toc subfiles when the main news file gets large.

[pajod]

(updated version now references unreleased 23.0.0 release)

@benoitc kind bump, this makes (at least properly rebased) PRs much more readable by avoiding having to merge settings.rst changes

[benoitc]

thanks!