Use jupyter-sphinx directives for markdown/rST pages

[choldgraf]

Right now the ipynb parser is the only thing that knows how to deal with outputs of running a cell. However, we could replicate similar behavior if we used machinery from jupyter-sphinx to run code and display outputs. I'm not 100% sure how this should happen, but this issue is to track the topic.

In particular,

if we could have code that did the same execution code that jupyter-sphinx uses (https://github.com/jupyter/jupyter-sphinx/blob/master/jupyter_sphinx/execute.py#L76)
but instead of directly inserting outputs we used the same OutputBundleNode that MyST-NB uses: https://github.com/ExecutableBookProject/MyST-NB/blob/master/myst_nb/parser.py#L136
then we could use the same CellOutputToNode transform that we use here: https://github.com/ExecutableBookProject/MyST-NB/blob/master/myst_nb/transform.py#L31

I think it'd be even better if jupyter-sphinx itself defined the CellOutputToNode object, and then Myst-NB could reuse that for both its markdown and ipynb uses. Maybe @akhmerov has thoughts on that

ping @rossbar who I think was interested in this issue

note: this is separate from the "two-way conversion between ipynb and myst nb markdown...in this case, the markdown will be directly read into Sphinx without being converted to an ipynb file first.

another note: we should consider a future where a jupyter-cache exists as well, as at some point we'l want to leverage that cache to avoid having to run all the code each time if it is in a markdown cell

[akhmerov]

I fully support the idea of incorporating this in jupyter-sphinx.

Sorry for lack of activity on my side: my bandwidth will stay saturated for a few weeks. However I'm happy to review/merge/mint a release for jupyter-sphinx.