math: add mathjax rendering

[McShelby]

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[McShelby]

Best option here would be to activate the goldmark-mathjax-extension, but this has to be done by Hugo itself which seems rather unlikley to happen in the near future. https://discourse.gohugo.io/t/use-goldmark-mathjax-extension/25721/26

There are other options, all with their own disadvantages of either having conflicts with otherwise valid markup constructs or being inconvenient for typing.

Clientside rendering also has the problem of detecting if a math equation is present and only load the huge mathjax dependencies in this case. For inline code fences there currently is no way to handle it in a render hook.

Further reading:
https://yihui.org/en/2018/07/latex-math-markdown/
https://www.stderr.nl/Blog/Blog/MathJaxInMarkdown.html

[McShelby]

Now on GitHub: https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/writing-mathematical-expressions