Server side rendering of math equations (KaTeX)

[deining]

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

Advantages:

• no JS needed for math content
• typesetting of formulae using LaTeX standard syntax, no escaping of delimiters needed
• no need to load javascript files at run time
• server side rendering (instead of client side rendering)
• internal rendering engine supports rendering of chemical equations out of the box (as of hugo version 0.144.0)
• no need to update KaTeX dependencies any more

Disadvantages:

• minimum required hugo version is now 0.141.0

I hope you like this PR.