Install Sphinx extensions before the build

[smashwilson]

Automatically install Sphinx extensions before building the content, like we already do for Jekyll. We'll likely just need to look for and pip install -r a requirements.txt file in the content repository, although it would be fancier to do it from code rather than shelling out.

[meker12]

I remember this. At the time, I didn't know how to make the requirements.txt file. Now I have one and know why I have it ;-)