🧱 Transform notebook blocks with captions to figures in place

[fwkoch]

This PR enables creating a figure directly inside a notebook by adding a caption to a cell:

#| caption: My figure caption
...
plt.show()

This modifies the block structure from:

block > code/output

to

block > figure > code/output

This PR also cleans up figures referencing other figures (e.g. if you want to include a "notebook figure" created as described above in a separate markdown page). Referencing a figure from another figure removes the original caption / figure number. If you want to keep the original caption / figure number, you may embed the figure instead.

For full functionality with thebe, this requires jupyter-book/myst-theme#272

TODO:

• Fix up embedding these figures to make sure they are computable
• Allow kind to be table
• Follow through caption parsing

There is still a little bit of wonkiness with resolving embedded content and figure references simultaneously, but those issues are mostly encountered in edge cases - I think for now this is good enough!