Document Geo Design

We need an architecture diagram such as the one we have for GitLab.com architecture to fully understand where the bottlenecks can be.

Let's start with this as a first step to complete Geo Documentation before we try to build anything (else it will be building things blindly).

We currently have some documentation in https://docs.gitlab.com/ee/gitlab-geo/ but it's not declaring how the different pieces fit together, which processes need to be run how, which ports need to be open, etc, etc.