Document how to use Jupytext as a python library

[choldgraf]

I'd like to build in support for Jupytext files within Jupyter Book (so content can be stored in a jupytext-compliant format and would be converted to a notebook at build time). However, I don't see any obvious documentation to use Jupytext as a Python library in the docs. Maybe I'm missing something? But if not, it would be useful to know how to use Jupytext from within Python.

[mwouts]

Hello @choldgraf. Well I like very much what you suggest - it would also be a great illustration for for #247 (and we could start with Elegant Scipy, a book that is already written in the markdown format).

In short, my expectation for using Jupytext as a Python library is that you should be able to replace nbformat.read with jupytext.read.

There is a short section on this at https://jupytext.readthedocs.io/en/latest/using-cli.html#reading-notebooks-in-python. But you're right, we should at least move that paragraph to a section of its own, and why not, expand it.