chore: server side rendering of math equations

[deining]

This PR brings server side rendering of math equations via hugo's built-in KaTeX rendering engine.

Advantages:

• server side rendering (instead of client side rendering)
• no JS needed for math content
• typesetting of formulae using LaTeX standard syntax, no escaping of delimiters needed
• no need to activate math mode in frontmatter any more
• internal rendering engine supports rendering of chemical equations out of the box (as of hugo version 0.144.0)
• no need to include KaTeX dependencies any more

I hope you like this PR.

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[imfing]

Hi @deining,

Thank you, this is great! I plan to merge it in the next minor version.

I may follow up separately with a small PR to make the katex CDN configurable