Clarify use-case and docs structure

[choldgraf]

This is a quick PR to clarify how documentation structure maps onto the structure of this theme, and to provide guidance for folks that want a single-sidebar theme instead. cc @zaneselvans who I think was confused by our docs - would this help?

[zaneselvans]

Hey thanks! Yes, this would have made it much clearer. My first intuition was that I had messed up the formatting or configuration somehow, or done something dumb that just wasn't being exercised on the RTD theme. Knowing that this is the expected behavior and that users might want to re-jigger the toctree into a small number of high-level sections with a number of sub-pages is definitely helpful.

[damianavila]

LGTM