Document / better support for separating user domains from the Hub domain

[minrk]

We have opt-in per-user subdomains to better isolate user servers from each other, but these are typically and by default subdomains of the Hub.

It's well established that when you have 'untrusted' user-controlled pages (like GitHub pages), they should be on their own domains, rather than subdomains. I believe it's already possible to do this with the new subdomain_hook to have the Hub on a different domain (e.g. jupyterhub.mydomain.edu and user.mydomainusers.com).

We should at least document how to put user servers on a totally different domain with what we have now, and probably make it easier to express this via config (e.g. instead of single JupyterHub.subdomain_host config, separate JupyterHub.hub_domain and JupyterHub.user_domain config).