Organize example notebooks

This branch does two things:

* I have moved our examples directory out of the docs dir, to make it top level and easier to find.
* I have refactored and rewritten the Notebook examples.

Questions:

* Some of the notebook examples still have a very tutorial feel that doesn't really match the idea of "examples".  But they still really do show examples of the notebook.  I am not sure how we want to organize this type of material - I still think the current org is lacking.  Would love ideas on how to handle it.