ENH: Build download notebooks using sphinx-tojupyter

[mmcky]

This PR adds sphinx-tojupyter and enables support for download notebooks.

These notebooks are built from the sphinx.ast and are modified from source notebooks in that:

1. they contain only basic markdown markup which makes them more readable in jupyter notebook or jupyterlab (for the time being until myst is rendered via javascript)
2. links are updated for context to point back to the website for external links (i.e. documents)