Unify and update documentation

[blthayer]

• Convert README file to rst
• Split README file out into different files so it can be included in the API documentation
• Include README content in API documentation
• Clean up and update documentation

[blthayer]

@mzy2240 @SmartJingJing123 - So that our documentation can be more unified, I'd like it all (with the exception of the JOSS paper itself) to be in restructure text. Here's a quick primer:
http://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html

[blthayer]

This is more or less done, and we have several different issues open for further tweaking the documentation.