Geo: Serve Pages from secondaries for the same domain as the primary

I.e. Requests for *.gitlab.io are handled properly by secondaries. Whereas it is not possible to serve other domains from secondaries, e.g. *.gitlab-secondary.io.

This is actually how we expect Pages to work on secondaries by default. In a sense, it is a feature, since you can route requests to the closest Geo node and "it just works". So this issue is about communicating this Geo + Pages behavior to customers.

• Add documentation as needed
• Consider opening an issue to add UX/UI work to make this behavior clear during set up of Geo + Pages

Additionally

If we do want to support other domains from secondary nodes, we should open a separate issue. That would take Geo effort to make happen, and is not particularly straightforward since as @nick.thomas mentioned here, https://gitlab.slack.com/archives/C32LCGC1H/p1549533347467600 it's kind of weird.

custom domains will need to choose whether to be served from the primary or secondary

On the other hand, @ashmckenzie mentioned that it

would be beneficial to have gitlab.io as a CNAME to gitlab-secondary.io for visibility / debugging

• So, consider opening an issue to support serving to different domains per Geo node
  • Is there customer demand?
  • Is it difficult for us to debug without this?
  • How would custom domains work?