Add Sphinx documentation generation.

TODO:

- [x] Close #32 as duplicate
- [x] Create Sphinx configuration files
- [x] Add Sphinx target to makefile
- [x] Determine which `*.rst` files to save
- [x] ["Sphinxify"][1] source documentation
    - [x] Convert docstrings to a format understood by `sphinx`. ([example][3])
    - ~~Add type annotations? (not sure if this is useful)~~
    - ~~Make class/module/function references hyperlinks ([example][2])~~ (let this happen naturally)
- [x] Have the version in conf.py be set using a call to doorstop to get the version
- ~~Get Doorstop linked to ReadTheDocs~~ (moved to #105)

[1]: https://pythonhosted.org/an_example_pypi_project/sphinx.html#function-definitions
[2]: https://pythonhosted.org/an_example_pypi_project/sphinx.html#python-cross-referencing-syntax
[3]: https://pythonhosted.org/an_example_pypi_project/sphinx.html#full-code-example