Help (and seeds): cover Mathjax so each affected community doesn't have to roll its own

[cellio]

meta:289449

The default help includes a topic on formatting. Because Mathjax has to be specifically enabled, the default help doesn't include that formatting, leaving communities that use it to add it (for example on Math).

It would be better if we had Mathjax help as part of the "core", with a note that it is not used by default but can be enabled. I don't have an opinion on whether it should be a section in the existing page (like in that example) or a separate topic.

When creating or editing this help, please address the question about escaping in the meta question that motivated this issue.