Replace MathJax with KaTeX

[jonaharagon]

Changes proposed in this PR:

• Replace MathJax with KaTeX, seems to be the preference these days: https://squidfunk.github.io/mkdocs-material/reference/math/?h=katex#comparing-mathjax-and-katex

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[dngray]

Yes, we prefer the MathML now that Chrome also supports it #2477 (which was the reason we used MathJax in the first place).

MathJax also had MathML output which is better for accessibility, so not sure we would have ever switched to Katex (we never had much math anyway) which was Katex's main advantage.