Documentation for Skyplane

[parasj]

This PR adds documentation in Spinx/ReadTheDocs format.

To build, run:

$ pip install sphinx-autobuild
$ sphinx-autobuild -b html docs docs/_build

[parasj]

Useful page with quickstart for docs https://www.sphinx-doc.org/en/master/usage/quickstart.html

[parasj]

Consider automatically building and pushing changes to documentation to a gh-pages branch: https://github.com/ammaraskar/sphinx-action