Clarify current Pages architecture and define what are we doing with that service

During the summit there was an idea floating around about leaving pages behind in Azure.

To do so, we first need to know what are the bare minimum things that are required to run gitlab-pages in isolation, to then decide if we can and should leave it behind.

So, I would like to get 2 things our of this issue:

1. An architecture diagram of the service explaining which are all the dependencies, metrics and SLAs, and how it interacts with the rest of GitLab itself
2. Enough information to make an informed decision, thus, a decision.

Assigning to @ayufan and @nick.thomas because they are listed as the maintainers