Draft: Add KaTeX support in the Markdown user guide

What does this MR do and why?

Implements #1610

The following docs were used:

• https://katex.org/docs/node.html
• https://katex.org/docs/browser.html
• https://katex.org/docs/autorender.html

Screenshots, screen recordings, or links to review app

https://axil-add-katex.docs.gitlab-review.app/ee/user/markdown.html#math

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.

2. Compile Nanoc and run the preview:

   make view

3. Visit http://localhost:3000/ee/user/markdown.html#math and make sure the math examples are rendered like https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/user/markdown.md#math.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.