Enable dask-gateway on opensci big binder

[yuvipanda]

Follow-up to #4247.

Dependent on #4240 being merged.

• Enable dask-gateway in the big binder instance (opensci, big-binder: enable dask-gateway #4317)
• Make appropriate changes to our documentation ([docs] binderhub and daskhub: scheduler/worker pods to have permission to pull built images #4316)

[yuvipanda]

This is unblocked now, as #4240 has been merged.

[consideRatio]

From #4317 (comment):

@GeorgianaElena @sgibson91 nice work on the instructions! Setting up daskhub in the newly setup big-binder in opensci cluster worked great!

I added instructions on the need to configure dask-gateway with image pulling credentials when binderhub has pushed to a private registry as well in #4316 - but beyond that, nothing else seems needed to handle the combination of binderhub based hubs + dask-gateway.

[consideRatio]

I've opened #4315 that I think is key for any demo of binderhub.

Also note that testing binderhub+dask-gateway can be done by launching the repo https://github.com/pangeo-data/pangeo-docker-images that will build the pangeo/pangeo-notebook image.