Rethink documentation

[rstacruz]

I think it's a problem that the repository for documentation mina-docs isn't getting maintained well. I think it should be consolidated into the main mina repository.

Furthermore, I'm not sure how I feel about maintaining separate files for documentation, when there's lots of inline documentation. I'm neither for or against it, though I personally appreciate having both the convenient inline documentation and the ability to make detailed documentation, but maintenance of both are becoming a bit of a burden.

Ideas:

1. Simply move mina-docs into the main repo. Pro: least work possible, Con: duplication of documentation with inline
2. Rewrite documentation in Sphinx (example of a Sphinx-made documentation). Pro: we'll be using a more-supported tool. Con: the site will look different.
3. Write a simple routine to extract inline code comments into separate pages.

[rstacruz]

cc @mikong for ideas