CI: check for jldoctest end markers and unknown admonitions

[benlorenz]

x-ref: #4321
cc: @fingolfin @lgoettgens

Edit: errors look like here: https://github.com/oscar-system/Oscar.jl/actions/runs/11879378622
The error should link to the corresponding docstring. Unfortunately I could not figure out how to find the exact line inside the docstring for the jldoctest / admonition.

If the errors are triggered from new changes these should show up as annotations in the Files changed tab as well.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.52%. Comparing base (fb767f2) to head (9e0f07b).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4325      +/-   ##
==========================================
+ Coverage   84.49%   84.52%   +0.02%     
==========================================
  Files         645      646       +1     
  Lines       85589    85631      +42     
==========================================
+ Hits        72321    72382      +61     
+ Misses      13268    13249      -19     

Files with missing lines	Coverage Δ
...imental/BasisLieHighestWeight/src/UserFunctions.jl	92.42% <ø> (ø)

... and 8 files with indirect coverage changes

[lgoettgens]

This job needs about 1 minute (+ building Oscar and deps). I think this is acceptable for the kind of tests it performs. And I would hope that the building time goes down once this is merged and makes use of the actions cache.

Once this is merged, I think it should be marked as a "required" job.