Revamped docs system

A couple of major changes:

- documentation of an example now lives in the example folder
- `build_docs` script assembles the example documentation and ipython notebooks and automatically creates a table of contents in `index.md`

The motivation:

- make examples and example notebooks easier to contribute: no need to touch docs/ at all
- make example documentation more easily usable: no need to fire up doc server, just look at corresponding readme
- a step toward more complete documentation of the code, automatically assembled from readme's and docstrings

@shelhamer take a look, comment to approve -- i can merge