Update HA architecture diagrams to include 3 PostgreSQL nodes
The HA architecture diagrams should be updated to include 3 PostgreSQL Database nodes to match the recommendation in https://docs.gitlab.com/ee/administration/high_availability/database.html#architecture. 3 nodes are recommended to help prevent repmgr
split-brain.
Diagrams that need to be updated:
- https://gitlab.com/gitlab-org/gitlab-ee/blob/master/doc/administration/img/high_availability/fully-distributed.png
- https://gitlab.com/gitlab-org/gitlab-ee/blob/master/doc/administration/img/high_availability/horizontal.png
- https://gitlab.com/gitlab-org/gitlab-ee/blob/master/doc/administration/img/high_availability/hybrid.png
Related to https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8659