Document how to support a sequence of hubs with an external home page

[yuvipanda]

We have been working on moving the UToronto series of hubs to something with a unified external home page that's managed by them, linking out to the hubs themselves via links. This is transparent to the end user, but allows us to more easily manage multiple environments as independent images without having to fiddle with user profiles. It also allows them to manage homepage + content their way, without any dependence on us.

This is the first time we are doing this, but I suspect this will be a useful pattern in the future too.

So far, it has involved:

1. Using /hub/user-redirect style URLs to link out to notebook / lab / rstudio as necessary in appropriate hubs
2. Possibly have to setup automatic login whenever the home page is hit, if appropriate
3. Possibly redirect users explicitly going to the home page to the externally managed home page.

We should document all this so it's a repeatable process.

[yuvipanda]

One consequence here is that just sending people to the staging hub home page doesn't work. we need to send them directly to the staging homepage login url - /hub/oauth_login. staging.utoronto.2i2c.cloud/hub/oauth_login works for example.