CI: adopt current GAP package workflows

[fingolfin]

Split the manual build into docs.yml, test against devel/latest/minimal GAP instead of a hard-coded list, update to actions/checkout@v7, and add the release and check-pending-release workflows from gap-actions.

Assisted-by: Claude Code (Opus 5.5)

---

Not sure if you are interested in the release workflow at all, but on the other hand you don't have to use it even if it is there.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.43%. Comparing base (c891fe6) to head (ff0236c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #54   +/-   ##
=======================================
  Coverage   89.43%   89.43%           
=======================================
  Files           5        5           
  Lines         899      899           
  Branches       30       30           
=======================================
  Hits          804      804           
  Misses         91       91           
  Partials        4        4           

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