Adds support for Mathjax

[EiffL]

This small PR fixes the import of the mathjax scripts.

This theme has a special feature which allows us to specify which scripts to loads as part of the layout files.

All we need to do is add this in layouts/base.html:

common-ext-js:
  - href: "https://polyfill.io/v3/polyfill.min.js?features=es6"
  - href: "https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js"

and these scripts will be loaded in all pages which inherit from the base layout