add mathjax extension

[glemieux]

The build still doesn't look any better. I think I'll need to dig into the mathjax, sphinx and rtd documentation about configuring mathjax

[glemieux]

The build still doesn't look any better. I think I'll need to dig into the mathjax, sphinx and rtd documentation about configuring mathjax

It looks like the issue was that I had forgetten to remove the imgmath extension, which was overriding the mathjax extension.

Here's before:

And here's with this PR now:

This is without adding any particular mathjax configurations. Definitely seems like an improvement to me. What do you think @rgknox?

[rgknox]

I think it's an improvement