Move the doctests into their own test group

[michakraus]

What

test/quality/doctests.jl moves from the group slow into its own group doctests. An empty ARGS runs core and slow, so Pkg.test() and every job of the CI test matrix no longer run the doctests. Pkg.test(test_args = ["doctests"]) runs them, and the CI Doctests job stays their runner.

Why

The test convention now has a sixth group, doctests. The doctests belong on one pinned job, as the comment in CI.yml says; the matrix ran them on every Julia version and every system as well.

Checks

• The layout check of the test convention: clean.
• The group doctests passes on Julia 1.13.1: the Test Summary reports 1 test passed of 1 in total.
• The changed test/runtests.jl is formatted, and NFC.
• CHANGELOG.md: one added line under [Unreleased].

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.13%. Comparing base (187854d) to head (586c12c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
- Coverage   91.39%   91.13%   -0.26%     
==========================================
  Files           9        9              
  Lines        1162     1162              
==========================================
- Hits         1062     1059       -3     
- Misses        100      103       +3     

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