point directly to colab notebooks

[proppy]

Instead of linking to developers.google.com/silicon I think it might be better to point directly to the colab version, ex: https://colab.research.google.com/github/chipsalliance/silicon-notebooks/blob/main/digital-inverter-openlane.ipynb, so that partipants can immediately open it.

Additionally we could include
https://github.com/chipsalliance/silicon-notebooks as a submodules under examples so that more examples are directly discoverable directly from the git repo.