Add documentation with sphinx.

[mrdrozdov]

This PR adds documentation using jekyll + sphinx.

A preview is available here: https://mrdrozdov.github.io/graphgrove/

Notes:

• The homepage is built using jekyll, and is automatically generated from the top-level README.md.
• The documentation is built using sphinx, and is automatically generated by class/method docstrings.
• Can run docs locally with make docs-serve, and publishes docs with make docs-deploy.
• For jekyll, ruby 2.7 is required. I recommend installing this with rvm. May need to run cd homepage && bundle install.
• For sphinx, look at installation information at docs/README.md.
• FYI, may want to add a git hook that automatically runs make docs-deploy every time when pushing a new commit. Otherwise can manually deploy.