chore: use KaTeX Sphinx plugin

[flying-sheep]

KaTeX renders to static HTML and doesn’t re-layout the page a fraction of a second after load.

compare (you might have to use devtools to see it if you have the page in cache):

mathjax: https://docs.scvi-tools.org/en/stable/api/reference/scvi.model.base.DifferentialComputation.html#scvi.model.base.DifferentialComputation.get_bayes_factors

katex-pre-render: https://cookiecutter-scverse-instance--237.org.readthedocs.build/en/237/generated/cookiecutter_scverse_instance.pp.elaborate_example.html

[github-actions]

A PR has been generated to the instance repo:
scverse/cookiecutter-scverse-instance#237

You can check out the PR to preview your changes
in an instance of the cookiecutter template. The PR will be kept in sync with
this PR automatically.

[grst]

Just one minor concern that it's one additional thing that can go wrong while building docs. But it's probably fine.

[flying-sheep]

I think it would take a truly ancient version of nodejs being present to break this.