Geo: Document how our Git push proxy works

Document how our Git push proxy code works, in order to be able to understand if any change is required for that to be implemented as part of the effort to run Geo in a Kubernetes cluster.

• Write documentation in doc/development

Revelent Isuses/MRs:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21034
• https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6456
• https://gitlab.com/gitlab-org/gitlab-ee/issues/6533
• https://gitlab.com/gitlab-org/gitlab-ee/issues/7405