Add a docs-build CI job

[brendancol]

Refs #3249 (part 2 of 6).

Sphinx currently only runs on Read the Docs, so a PR that breaks the docs build isn't caught until after merge. This adds a docs workflow that runs on PRs touching docs/**, README.md, setup.cfg, or .readthedocs.yml:

• installs .[doc,tests] on Python 3.12, mirroring the RTD pre_build step so the two can't drift
• installs the pandoc binary via apt (nbsphinx needs it; the pandoc pip package doesn't ship it)
• sphinx-build -b html -j auto, gating on errors only (no -W; the tree has a handful of pre-existing notebook/reference warnings). The read phase is dominated by plot-directive examples in the reference docstrings (sample data load + numba compile per page); every extension in conf.py declares parallel_read_safe, so -j auto cuts the build from ~8 minutes to ~3.5.
• linkcheck runs on a weekly cron (Mondays 06:00 UTC) and workflow_dispatch instead of per PR. It cost ~10 minutes per PR, never gated (continue-on-error), and was in fact already exiting 1 with nobody noticing. The cron job drops continue-on-error so a broken link actually fails the run. Expect the first scheduled run to be red: there are real broken links to fix.

The job id is docs-build, distinct from the pytest workflow's run job, to avoid the check-name dedup problem that hides failures.

Test plan:

• YAML parses
• docs-build passes on this PR in 4m51s (was 18m48s)
• linkcheck is skipped on pull_request events

[brendancol]

PR Review: Add a docs-build CI job

Blockers

None.

Suggestions

• .github/workflows/docs.yml: the job has no timeout-minutes. continue-on-error: true keeps a linkcheck failure from gating, but a hung external link check still occupies the runner until the default 6-hour job timeout. A cap around 30 minutes covers a full build (RTD finishes in well under that) and bounds the damage.

Nits

None.

What looks good

• Action versions match test.yml (checkout@v4, setup-python@v5), and the install step mirrors .readthedocs.yml's pip install '.[doc,tests]' exactly, so CI and RTD build from the same dependency set.
• The paths filter includes the workflow file itself, which is why this check ran on this very PR.
• Job id docs-build avoids the duplicate-check-name dedup problem already seen between the pytest workflows.
• apt pandoc install is needed: the pandoc pip package in the doc extra does not provide the binary.

Checklist

• Correctness/backend/test items: not applicable (CI config only)
• Verified the workflow triggers and the YAML parses

[brendancol]

Follow-up to the review: added timeout-minutes: 30 to the docs-build job so a stalled linkcheck can't hold a runner for the default 6-hour timeout. No other findings to address.

[brendancol]

Measured the first two runs: ~26-27 minutes each (HTML build ~11, linkcheck ~14), which left under 3 minutes of headroom against the 30-minute cap. Raised it to 45.