Add automatic testing of Jupyter notebook files

[mdko]

It would be beneficial to automatically verify that the Jupyter notebook examples still run properly at all times.

For examples, problems occur if PyRTL files are renamed or moved and no longer accessible like before if a Jupyter file depended on this submodule access. This shouldn't be a problem normally, since almost everything you'd want to use from PyRTL should accessible via pyrtl.*, but could cause problems in the future with examples that access rtllib. For example, estimate.py was recently renamed, and a PR for example7-synth-timing-draft.ipynb will be made soon to fix it.

[mithro]

You might also want to checkout tuttest which lets you run your documentation in CI. We use it in the SymbiFlow Examples repository, see https://github.com/SymbiFlow/symbiflow-examples/blob/master/.github/scripts/install-toolchain.sh for example.