Setup readthedocs

[trey-stafford]

Setup RTD for the ogdc-runner. Use autodoc to generate API Docs.

As a first step, this RTD site can just have the content we've already developed (README + contributing info), plus the API docs.

Longer term we want to host information about e.g,. developing recipes, how-tos, tutorials, etc. We expect this to be a heavily-used reference during the dev hackathon