Pdpinch/documentation updates

WIP. Not ready to merge yet. 

@cgee or @bdero can you review the installation instructions, esp. part 3:

https://github.com/mitodl/edx-sga/blob/pdpinch/documentation-updates/README.rst#installation
Could you use h2 headers for these subsections ("Installation", "Course Authoring in edX Studio", etc)? (------------------ instead of ==================)