Add section / links to Dask

[SammyAgrawal]

I think we should add doc sections on how and when the Hub is using dask under the hood and either write or (more likely) link to/ reference ways to optimize dask xarray.

• How to open up dask scheduler and see how resources are being utilized

[jbusecke]

This should be explained somewhere upstream (e.g. the pangeo docs, but they are hopelessly outdated). As an intermediate solution I would suggest adding some text and linking to the xarray docs for now.