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 runner reported a pass in 28 s, with the Test Summary "selected | Pass 1 Total 1 16.4s".
• The changed test/runtests.jl is formatted, and NFC.
• CHANGELOG.md: one added line under [Unreleased].

Open pull requests that also change test/runtests.jl: #338 Add LatentVectorFieldLoss: the reduced vector field against the pushed-forward full one, #337 Add ReducedHamiltonianLoss: fit a decoder to states and reduced-Hamiltonian gradients, #336 Add StandardAutoencoder, an autoencoder of Dense layers, #329 Add PODArch for proper orthogonal decomposition, #321 Check the Poincaré integral invariant of a decoded loop, #310 Train through GeometricOptimizers' TrainingOptimizer (G4), one method per leaf, #207 Parametric Generalized Hamiltonian Neural Networks. The later one to merge rebases.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.64%. Comparing base (d7c8960) to head (22d67e9).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #340   +/-   ##
=======================================
  Coverage   86.64%   86.64%           
=======================================
  Files          76       76           
  Lines        2786     2786           
=======================================
  Hits         2414     2414           
  Misses        372      372           

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