Add Datashader example to dask-examples

During a recent Dask tutorial someone asked "Can Datashader run on Dask?" and I was pleased.

It would be good to have an easy-to-run example that was advertised Datashader from Dask resources. Often we do this in examples.dask.org . Is there an example that makes sense to deploy there?

Start by reviewing the request for an easy-to-run Datashader example that can be advertised through Dask resources, especially examples.dask.org. Identify a suitable example and determine how it would be deployed there; done means the example runs easily and is advertised through the requested Dask resource.

Written by the indexing model from the issue text.