CI: Test building docs in parallel on Cirrus CI

[scottshambaugh]

PR summary

Just experimenting for now, curious if this speeds things up

For comparison, this docs build took 20m 11s as a baseline on a single core.

PR checklist

• [n/a] "closes #0000" is in the body of the PR description to link the related issue
• [n/a] new and changed code is tested
• [n/a] Plotting related features are demonstrated in an example
• [n/a] New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[scottshambaugh]

With 2 cores the time to build the docs dropped to 18m 9s.

The errors that caused this failure are:

WARNING: the pydata_sphinx_theme extension is not safe for parallel writing
WARNING: doing serial write

This is the relevant issue: pydata/pydata-sphinx-theme#1643. Some users there reported no problems with setting the parallel_write_unsafe flag to False. Will try that next.

[scottshambaugh]

Ok digging into it more, looks like this was talked about in #27658

The parallel unsafe flag was added in pydata/pydata-sphinx-theme#1642, where this was the behavior:

marks the theme as unsafe for parallel_write. This is based on extensive testing of the SciPy docs build, where the API pages were reliably getting the wrong left sidebar TOC when built in parallel, and getting the correct sidebar TOC when built with a single job. After marking the theme as parallel-write-unsafe, the SciPy docs now end up with the correct sidebar on all pages even when building in parallel (i.e. when pages are read in parallel but written serially)

Worth another look when that all gets fixed but closing for now.