Add new notebook for diagnostic tools #267

It'll be useful to have a dedicated notebook to discuss Dask's diagnostic tools, including task graph visualization, widgets for local compute, dashboard for distributed computation, etc. It can be a subset of https://docs.dask.org/en/stable/understanding-performance.html

@charlesbluca, @ncclementi, and I are submitting a Dask proposal for PyData Global 2022, and we plan to add this notebook as a part of our prep.