Document (in Sphinx) our markdown spec

This is a part of this overall project:

https://github.com/jupyter/nbformat/projects/1

and is a follow on to #95 that will create a sample notebook. In this issue we need to write documentation in the nbformat repo using Sphinx about the markdown we support. That guide should take CommonMark as a starting point and describe what we do and don't support as well as other extensions.

@ashutoshbondre @mpacer

Review issue #95 and the nbformat repository's existing Sphinx documentation, then use CommonMark as the reference point. Document which Markdown features nbformat supports, which it does not, and its other extensions; the guide is done when those differences and extensions are clearly described.

Written by the indexing model from the issue text.