Create upgrade guide for moving to larger Reference Architectures

As seen in gitlab-org/gitlab!108855 we don't cover the path in detail for moving to a larger Reference Architecture from a smaller one.

Generally it's just a straightforward task but for some components there is some prep work required when switching to their "HA" model, including:

• Gitaly to Gitaly Cluster
• Redis to Redis Cache / Redis Persistent
• Postgres to Postgres HA (w/ Consul and PgBouncer)

These areas are probably best tackled in a separate guide detailing the one off actions a user would need to do (specifically this should be just a high level guide that links to the specific docs like Migrate to Gitaly Cluster as we want to stay DRY). Task is to do just that.