CI: check translated languages and fail on warnings

[suelje]

Why

Broken markup in a translation only shows up when Sphinx builds that language. The shared workflow builds English only, so a Weblate PR can merge a translation like

:doc:`tipi`          (source: :doc:`types`)

and nobody notices until the rendered page shows a broken link or raw markup. Read the Docs does build the translated languages, but with fail_on_warning disabled it reports success anyway.

What

docs.yml, the reusable workflow used by all three documentation repositories:

• New step Check Translations, behind the new input check_translations (default false). For each language in translation_languages it runs the dummy builder with -W --keep-going, so warnings fail the job but every language is still built. The step ends with a compact list, one line per broken string, plus an error annotation each:
• actions/checkout, actions/setup-python and actions/upload-artifact moved to their current majors (v7), which run on Node.js 24. GitHub warns about the Node.js 20 versions in every run.

Verified

• Tested against a fork with the admin documentation: de, pt_BR and sr pass, it fails with exactly the one remaining broken string.
• The same command run locally against all 29 catalogs of the admin and user documentation produces the same warnings as a full HTML build of each language.

[ralf401]

Nice, thanks! Question for my understanding: is it right that admin- and user docs have to get a check_translations: true flag afterwards?

[suelje]

yes right, this is an optional workflow step for now. I think it makes sense also because each documentation repo can define which translations to check (next to english)