Currently, sphinx allows customizing MathJax by passing a config dictionary for mathjax3_config

However, this has limitations, for example passing custom JavaScript preprocessing such as described here is not straightforward. For some discussion, see here: https://github.com/orgs/sphinx-doc/discussions/13147.

I propose that mathjax3_config should be allowed to, in addition to None or dict, be a single str that points to a .js file holding the custom mathjax config.