FEAT: Support to specify data or support files for a notebook

[mmcky]

python_essentials

• @mmcky will need to review handling of static files here. We would ideally like the _static/us_cities.txt file to be available at the notebook level for this situation.

I think what we should do is add this to .. jupyter:: to specify a file dependence. In the case of a local dependency (such as a data file) we could add:

.. jupyter::
   :attachment: <reference to file>

probably shouldn't use this term as it is something we can support for markdown cells (https://nbformat.readthedocs.io/en/latest/format_description.html?highlight=attachment#cell-attachments)

or

.. jupyter::
   :file: <reference to file>

[mmcky]

This has been implemented in #250. Not through a jupyter directive but as a dependency list in the conf.py file.