minor doc cleanup; parallelize tests

[sbailey]

This PR started out as a minor cleanup for building docs in prep for a new tag, but morphed into also adding parallelism safety for tests (run with pytest-xdist via pytest -n 8).

[Copilot]

Pull request overview

This PR improves documentation build reliability and makes several unit tests safe to run in parallel (e.g., under pytest -n 8) by isolating filesystem output and environment variables per test.

Changes:

• Refactor multiple tests to use per-test (or per-class) temporary directories and restore environment variables in teardown.
• Fix/cleanup minor doc-build issues (Sphinx Makefile default, additional mocked imports, and an invalid-escape warning in a Matplotlib label).
• Expand CI docs job to also run make html in addition to a direct sphinx-build invocation.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
py/desisim/test/test_quickgen.py	Move test I/O + env setup to per-test temp dirs for parallel safety.
py/desisim/test/test_quickcat.py	Write generated FITS files into an isolated temp directory and cleanup via TemporaryDirectory.
py/desisim/test/test_pixsim.py	Isolate env + output paths per test using TemporaryDirectory.
py/desisim/test/test_obs.py	Isolate env + output paths per test using TemporaryDirectory.
py/desisim/test/test_io.py	Create per-test temp dir + unique output filename; restore env in teardown.
py/desisim/test/test_batch.py	Generate batch script in a per-test temp dir; restore env reliably.
py/desisim/spec_qa/redshifts.py	Use a raw string for a Matplotlib label to avoid invalid escape sequence warnings.
py/desisim/qso_template/desi_qso_templ.py	Remove unused imp import.
doc/conf.py	Mock additional desitarget submodules during autodoc to avoid doc-build import failures.
doc/changes.rst	Update changelog entries/links for unreleased version notes.
doc/Makefile	Default SPHINXBUILD to sphinx-build (more portable default).
.github/workflows/python-package.yml	Add CI step to verify make html doc build path.

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[coveralls]

coverage: 44.352% (+0.004%) from 44.348%
when pulling 585dc4d on docs
into b97e17d on main.

[weaverba137]

I had a few questions that may lead to minor changes, but overall looks good.

[weaverba137]

One more question: do we know how many cores are available when running GitHub Actions?

[sbailey]

One more question: do we know how many cores are available when running GitHub Actions?

I have not found any specific mention of multiple cores available to GitHub actions; their primary parallelism appears to be splitting the various jobs to e.g. test different versions of python in parallel. Regardless, adding test parallelism safety makes testing locally much much faster even if GitHub remains single-core per pytest invocation. FWIW I've included pytest-xdist in the draft next release of the desi software environment at NERSC (currently "test-main2").

[weaverba137]

I think this is ready to go. Thanks for the changes.