Organization migration: Update design doc with diagrams and more design details

Problem

https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/cells/migration/ is very high level. It is also hard to quickly understand how it will interact with the rest of the application and infrastructure. Especially for those outside the team.

Proposal

Based on the outcome of Investigate Org Mover Design (#504180 - closed) and Org Mover design initial discussion the Group Tenant Scale offsite:

• Make the roadmap much more detailed
• Add diagrams
• Add more design details
• Capture decisions that have been made since it was last updated
• Add any further limitations for the MVC (Move an internal org on .com)