Add API docs using Sphinx

[michaeldickens]

Implementation of issue #44.

Changes:

1. Create Sphinx docs in doc/.
2. Use sphinx-apidoc to auto-generate API docs for all modules in docs/source/reference/.
3. Port over README.md (with some slight rewrites) to docs/source/index.rst, installation.rst, and usage.rst.
4. Generate HTML from docs at docs/build/html/.

To build docs:

1. cd doc
2. make buildapi
3. make html

To view docs, open docs/build/html/index.html in your browser.

(Note: Makefile was generated by Sphinx, but I added the make buildapi based on this StackOverflow answer.)

[michaeldickens]

I forgot, I need to update Poetry to add a dependency for pydata_sphinx_theme

Edit: fixed

[michaeldickens]

I am not sure whether the doc/build/ directory should be tracked, I noticed that scipy does track their doc/build/ but it contains binaries and stuff that git doesn't deal with well.