Add minimal ReadTheDocs / Sphinx site

[willingc]

Autodocument configuration options.

[rkdarst]

I was going to begin this, following examples of the other jupyterhub team projects. I've set up sphinx many times, so I'm not very worried. Since it will rearrange the current docs, this will affect almost all other PRs that come in the future. I think this is a good idea, but does anyone have objections to doing this?

@mbmilligan, if we do this, can you keep things flowing so that we don't get a huge mess?

[mbmilligan]

That sounds great, I agree it's worth having a documentation entry point along the lines of what the other JH subprojects have.

I'm happy to keep things flowing and handle fixup on PRs if needed. What mess specifically are you concerned about?