Add full API to documentation

[effigies]

Even if we don't have full documentation for every class yet, I think having a searchable API might be useful to some users, and it should be a pretty low cost effort with Sphinx. It will also help highlight which areas off the docs need attention.

For fmriprep.interfaces, may want to reuse nipype's population of inputs and outputs from the specs, though possibly with a little bit better formatting.

[oesteban]

I was thinking of creating a sphinx extension for nipype interfaces. But it's just a wish, for this PR it makes sense just reusing nipype's current code.