Add KaTeX to render math equations
We don't really need to render math equations, but there's one page that needs it. It's the Markdown user guide, and specifically this section https://docs.gitlab.com/ee/user/markdown.html#math.
Let's add https://katex.org as a dependency, and if possible, make it so it's only loaded on the Markdown guide.
Discussion started in gitlab#403710 (closed)
See also: https://discourse.gohugo.io/t/katex-in-hugo/43274/19
Edited by Sarah German