Reorganize docs slightly

[kyleabeauchamp]

Re #271

So I think the current "main" tutorial is pretty good.

Maybe for the second tutorial, we should just do Ward / alternative clustering / RMSD choices.

Then, we could have a section of "Additional Tutorials" that might include tICA, BACE, SCRE

Obviously the choice of which tutorials belong in the "main" section and which ones are "additional" is up for discussion. In particular, tICA might very well be a main tutorial.

To summarize, here's my current thinking:

Tutorials

1. Main workflow (Cluster, Assign, analyze)
2. Alternative Clustering
3. Alternative analysis methods (TPT)

   Additional Tutorials (somewhat but not too much hidden)

4. tICA
5. BACE
6. SCRE

[kyleabeauchamp]

So by "hiding" the additional tutorials, perhaps what I mean is that we have a single link "Additional Tutorials" that brings users to a page with a list of tICA, BACE, and SCRE tutorials.

[rmcgibbo]

I'm happy to merge anything, but I don't really know much about how sphinx works. I've been mostly going by trial and error. If you go into docs/html, you should be able to build/rebuild with make html. To get the theme right, you'll have to pip install -r docs/sphinx/requirements.txt