Document how to use as a Python library

[choldgraf]

Currently the README focuses on the CLI use-case for Jupyter-Cache. I think that we'll probably want to use the Python API when using it as a part of a build system. We should document this part of the user-facing API.

[mmcky]

@choldgraf -- @chrisjsewell and I had a discussion on this topic yesterday. I will meet up with @aakash today to discuss ways we can link myst-parser and myst-nb to get the outputs added to notebooks. We will look at the python api rather than the cli -- but @chrisjsewell suggested looking at the CLI to understand how the python api is put together.