Better documentation: readthedocs.io or similar

[zcutlip]

I'd like better documentation for this project.

The main use cases of pyonepassword are pretty straightforward but increasingly there are edge-case-ey scenarios that aren't obvious, e.g., having a service account token set while passing a password into OP(), which is considered an error.

Currently documentation mostly consists of:

• README.md
• docstrings on (most) of the API
• examples in the examples/ directory
• A couple extra things in the docs/ directory

These things are useful as they are, but I'd like to have something a bit more complete and centralized, such as a readthedocs.io site.

I don't really know how that works, but expect this would require:

• cleaning up the internal python docstrings,
• plus writing/maintaining some other documentation artifacts
• and then probably some sort of build scripts or similar