Add mathjax_config_file to load MathJax configuration from a file

[randolf-scholz]

Fixes #14023

Note: git was telling me that these two files were modified, I'm not sure what's going on:

(sphinx) rscholz@Z890:~/github/CONTRIBUTING/sphinx$ git reset --hard
error: failed to encode 'tests/roots/test-root/wrongenc.inc' from UTF-8 to latin-1
error: failed to encode 'tests/roots/test-warnings/wrongenc.inc' from UTF-8 to latin-1
HEAD is now at e5cf9e0da allow mathjax3_config to point to a JS file
(sphinx) rscholz@Z890:~/github/CONTRIBUTING/sphinx$ git status
On branch mathjax_custom_js
Your branch is up to date with 'origin/mathjax_custom_js'.

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git restore <file>..." to discard changes in working directory)
        modified:   tests/roots/test-root/wrongenc.inc
        modified:   tests/roots/test-warnings/wrongenc.inc

[AA-Turner]

Looks good based on a desk review (on mobile)!

I've left a few comments, but all minor.

A

[mgeier]

Is this really necessary?

As I said in #13147, the reason why it doesn't work is MyST-Parser, not Sphinx.

[randolf-scholz]

Is this really necessary?

What a weird thing to ask for a feature PR. Almost no features are strictly necessary. They are added because they are useful. This feature is useful because it solves a problem (#13147) without affecting 3rd party extension compatibility.

[AA-Turner]

@randolf-scholz I think we can resolve these concerns in the extra docs I suggested in #14023 (comment), also noting that MyST does non-standard things with MathJax config.

A

[randolf-scholz]

@AA-Turner It's still unclear to me what exactly you want me to add in terms of documentation and where to put it. My latest commit adds a (blue) info box:

And also .. versionchanged:: 8.3 Now also accepts a string with the path to a JavaScript file. to the mathjax3_config description.

[randolf-scholz]

I added mathjax4_config in this box in anticipation of #14080.

[AA-Turner]

Following @mgeier pointing out MyST's monkeypatching, I note that changing mathjax3_config to a str would break MyST: https://github.com/executablebooks/MyST-Parser/blob/8a44f5d35197b19aab2f1fe35b6f1dce4960bce5/myst_parser/sphinx_ext/mathjax.py#L67

As such, I've adjusted this PR to instead introduce a new config option, mathjax_config_path.

A