Execute jupyter notebook in CI and upload resulting html as build artifact

[jleibs]

Testing the notebooks is important and visual inspection of the notebook gives us some decent coverage of an end-to-end use-case on web.

This is relatively easy to do:

jupyter nbconvert --to=html --ExecutePreprocessor.enabled=True examples/python/notebook/cube.ipynb

We can upload the wasm to a new app.rerun.io/pr/xyzw target so we retain a copy for each PR, which will be helpful in general without wasting as much storage as every commit in the PR.

This is incremental work on top of (or maybe as part of):

• Restructure our CI workflows #1704