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

Disadvantages:

• minimum required hugo version is now 0.141.0

I hope you like this PR.