MathJax version 3 support

[chrisjsewell]

sphinx version 3 uses by default version 2, but in sphinx v4 will move to version 3 (see https://www.sphinx-doc.org/en/master/usage/extensions/math.html#confval-mathjax_config), which provides a significant increase in load time.

Adding this to our docs build generally works fine:

jupyter-book/docs/_config.yml

Line 40 in a96ece4

mathjax_config:

However, the mathjax_config TeX macros config no longer works, so this must have been altered in some way?

[chrisjsewell]

closed in preference for #963