Update docs theming to match mc-stan.org

[WardBrian]

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

Updates Sphinx config to match theming on https://mc-stan.org/

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):
Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[cdc-mitzimorris]

nice!

[amas0]

Confirmed the docs build and look correct on my machine.

One very minor point: the docs on mc-stan.org will default to using a light theme, whereas I think this build defaults to using whatever is set up as your default theme on your personal system/browser (which is dark on mine).

Not sure if there's a desire to align that behavior, just wanted to point it out.

[WardBrian]

Fair enough. It's fairly easy to control, depending on which we'd prefer to centralize around:

• if we want to default to light mode, we can just set "default_mode": "light" in the conf.py here
• if we want to use the system config, we can set respect-user-color-scheme: true in the quarto config of the mc-stan.org to get the same behavior over there