The MathJax help link (see below) presently links to An introduction to using $\TeX$ in the Harvard Mathematics Department.
Whilst this gives thorough treatment, it takes quite a bit of clicking to find the relevant maths stuff.
The quick reference provided at Mathematics.SE, however, is a more condensed, and therefore accessible, version of what anyone using MathJax at Engineering.SE might need. Their sidebar on the "Ask Question" page points to this more accessible version rather than to the Harvard manual.
I'd suggest that the MathJax help link on our site be changed to point to the Mathematics.SE version as well.
Or perhaps that it is copied/adapted to a similar page on Engineering meta and the link changed accordingly.