Geo docs: Describe SSF sync and verification workflow

Problem

Diagnosing #299819 (comment 822629467) was difficult. There is a lot that needs to happen to sync and verify a replicable, so the internal workflow takes a long time to follow.

Proposal

• Add an outline of the SSF replication and verification workflow in the Geo development doc: https://docs.gitlab.com/ee/development/geo.html
• Consider using mermaid diagrams

Consider modifying the following sections:

• https://docs.gitlab.com/ee/development/geo.html#repository-replication
• https://docs.gitlab.com/ee/development/geo.html#uploads-replication
• https://docs.gitlab.com/ee/development/geo.html#self-service-framework