Cells: Describe Cell 1.0: Support new enterprise customers

What does this MR do and why?

Cells: Describe Cell 1.0: Support new enterprise customers

This describes an architecture where we selectively proxy, and replicate information to support a particular customer on GitLab.com SaaS.

• Cell 1.0: Fully functional Cell deployed to GitLab.com SaaS, that can serve new enterprise customers.
  • Cell 1.5: Is build on top of Cell 1.0 architecture and is meant to provide tooling to migrate existing customers to Cell 1.0.
• Cell 2.0: Is meant to represent an ability for user to interact with many organizations and be able to serve public/open-source workflow on GitLab.com with Organization model.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/435034.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.