When the user is asking or editing a question, a "MathJax help" link is present on the right-hand side of the screen. The link is to http://www.math.harvard.edu/texman/
Currently, this only leads to a "Page Not Found" screen.
There are Wayback Machine archived copies of the page, for example https://web.archive.org/web/20190118085736/http://www.math.harvard.edu/texman/ - perhaps the link could be updated to point to that?
Thanks,
Astrid.