MAINT: update the list of sphinx math extensions, and use mathjax by default

This PR also uses a sphinx literal code import to show the list of extensions, rather than copypasting that part of the conf.py file into extensions.rst.  This should prevent the two lists from diverging.

closes https://github.com/matplotlib/sampledoc/issues/5
oh, i see what you did - this makes it hard to read the docs unrendered...