Some documentation is related to JupyterHub instead of repo2docker

[rgaiacs]

I read "Use a community-maintained image in a JupyterHub" and my impression is that it is only related to JupyterHub. I propose to migrate it to https://jupyterhub.readthedocs.io/en/stable/.

[manics]

We already have https://z2jh.jupyter.org/en/stable/jupyterhub/customizing/user-environment.html#choose-and-use-an-existing-docker-image which covers a lot of that.