allow mathjax3_config to point to a JS 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