The "MathJax help" link in the "LaTeX" section of the Markdown Editing Help page currently leads to https://legacy-www.math.harvard.edu/texman/ – but that page doesn't actually talk about MathJax.
I believe this post on Math.SE's Meta site is a much better link.
(A staff member on Meta Stack Exchange suggested creating a question here to request this change, and then "site mods can escalate issues from their Meta site by adding the [status-review] tag".)
/editing-helpon a site) displays different sections depending on which relevant site settings are enabled on a site, and the LaTeX section is only displayed on sites with LaTeX enabled. But there's actually an additional site setting that determines which URL the "MathJax help" link at the bottom of that section points to... $\endgroup$