ci: run docs tests on every PR

[hexeberlin]

Moves the docs test job into its own workflow (docs-test.yml) without path filters, so it runs on every PR and push to main. The build/deploy/link-check jobs remain path-filtered to docs/** changes in the original docs.yml.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.44%. Comparing base (40c4caa) to head (e49bc39).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2746   +/-   ##
=======================================
  Coverage   97.44%   97.44%           
=======================================
  Files          39       39           
  Lines        1838     1838           
=======================================
  Hits         1791     1791           
  Misses         47       47           

Flag	Coverage Δ
unit	97.44% <ø> (ø)

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