* WIP Initial commit of refactored notebooks section.

* Minor additions.

* Finished draft of notebooks quickstart.

* Renamed some files and clarified the notebooks setup doc.

* Updated the guide to custom notebook images.

* A couple more screenshots plus text refinements.

* Fixed references to JupyterHub throughout the docs.

* Updated in response to review comments.

* One more link fix.

* Clarified auth in custom notebooks doc.