add sphinx support

[tyarkoni]

No description provided.

[ljchang]

@tyarkoni: I recently updated my documentation for nltools and found a couple of new things that I found very helpful. First, if you're planning on using readthedocs again, there is a new beta for using conda which works way better and you don't need to deal with Mock for all the packages that have c-libraries. Second, sphinx-gallery is really helpful for making tutorials and it autobuilds jupyter notebooks to download. I believe it's developed by the scikit-learn/nilearn team. I should be able to help with this in a couple of weeks, but this has made things way easier for me. I sort of dreaded dealing with this before.

[tyarkoni]

Thanks, I'll keep this in mind when I get back to this!

[qmac]

Closing after #228