Use sphinxcontrib apidoc for autogenerated API docs

[djhoese]

I've had a lot of luck using sphinxcontrib-apidoc to automatically generate the API .rst files when the sphinx documentation is rendered and it has saved me a lot of time. I don't know if any other pangeo projects use it, but I'd be willing to add it here if no one has a problem with that.

[maxrjones]

Hi @djhoese, sorry that this issue went without a response for so long! Documentation improvements are always appreciated - if you still think this would save time you are welcome to open a PR.

[djhoese]

Yeah, I think this tool is always pretty helpful. Given my involvement with the Pangeo ML/NASA project I should be able to find time this week to tackle this. I'll add it to my TODO list. Thanks for responding to old issues!