Add support for KaTeX math formula

[stepnivlk]

Hi, first of all thx for this cool template.

I, as @m4dh0rs3, would like to use katex formulas, so I prepared this small PR. It's mostly a copy of https://www.getzola.org/themes/even/ with 3 changes:

• Bumped katex version to 0.12
• Changed config keys to extra.katex.*
• Added support for $ $ inline syntax

You can find some rendered formulas here: https://ktsk.xyz/posts/mu123-unit3/

Original request: https://github.com/RatanShreshtha/DeepThought/issues/7

[stepnivlk]

@RatanShreshtha done

[RatanShreshtha]

Thanks @stepnivlk