Render mathematical expressions in docs

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We want to be able to render math expression in our docs (Scaladoc as well as Laika docs), like PyTorch does with MathJax:

For Scaladoc there is an open issue already scala/scala3#15433 but I haven't gotten around to work on it yet. For Laika it should be easier as we control the template so we could try to embed MathJax or Katex there.

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Done for Laika (introductory docs) in #12 with KaTeX, but still needs to be done for Scaladoc.