Use sphinxcontrib-apidoc extension for readthedocs builds

[JWCook]

This PR replaces the run_apidoc event handler with the sphinxcontrib-apidoc extension. It does pretty much the same thing, but it's a little more convenient. Feel free to close this PR if you don't find that useful, but personally it's saved me quite a few headaches.

Notes:

• apidoc_separate_modules is equivalent to the -e option
• apidoc_module_dir and apidoc_output_dir are relative to the documentation source dir (/docs)
• I updated tox.ini so local Sphinx builds will use the apidoc config in conf.py instead of separately running the sphinx-apidoc command
• I added a .readthedocs.yml to install docs-requirements.txt. If you already have that configured in your settings on readthedocs.org, that file can be removed.

[jparise]

• I added a .readthedocs.yml to install docs-requirements.txt. If you already have that configured in your settings on readthedocs.org, that file can be removed.

Yes, we already have this configured on readthedocs.org. I think we don't need the additional configuration file.

[JWCook]

• I added a .readthedocs.yml to install docs-requirements.txt. If you already have that configured in your settings on readthedocs.org, that file can be removed.

Yes, we already have this configured on readthedocs.org. I think we don't need the additional configuration file.

Okay, I removed it.

[jparise]

This looks good. Thanks for the improvement!

[JWCook]

You're very welcome. Thanks for the great library!