Improve navigation in documentation

[NOTE: this is a PR against our provisioning branch, and should be merged while that branch is still outstanding.]

Some areas where we could improve:

- change to a theme that provides next/previous links and breadcrumb trails
- implement a nested toctree that implements a rational organization of the doc
- consider producing a linear version of the docs (e.g. LaTeX or ePub) and verifying that it reads from beginning to end in a sensible order