[math] Improved HTML rendering using KaTeX

[mmcky]

We should look at switching from mathjax to katex to improve math rendering speed

https://katex.org/

[mmcky]

Thanks @jstac

Had a Quick Look and it unfortunately won't be a simple drop in replacement.

https://67b6662495f36929ec2ce18c--taupe-gaufre-c4e660.netlify.app/linear_equations

The server side rendering is really cool, can use node to build the math during the build phase which means the math loads straight away but the formatting is all off and the docs doesn't appear to have a simple fix for this.

Will touch base with the jupyter-book team re: rendering in mystmd.