Cells 1.0 blueprint - Add process for migrating orgs

What does this MR do and why?

This MR describes the process of moving select top-level groups into a secondary cell at Cells-1.0. It also describes how we will eventually reconcile the top-level groups into a single GitLab organization at Cells-1.5.

At Cells-1.0 we want to move some top-level GitLab groups from the primary cell onto a secondary cell. This allows us to start dogfooding the solution. In order to achieve we plan to create a temporary organization called GitLab Inc on the secondary cell and move select GitLab top-level groups to this organization using direct transfer.

At Cells-1.5 the remaining GitLab top-level groups on the primary cell will be moved to the GitLab organization. We will use org mover to move the GitLab organization from the primary cell to the same secondary cell as the GitLab Inc organization. We will then consolidate all top-level orgs into the GitLab organization and delete the GitLab Inc orgnization.

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