Add Documentation for Managing Multiple Projects

[nachocano]

Problem
There is currently no documentation on how to handle multiple projects.

Persona:
Developer

Exit Criteria
A measurable (binary) test that would indicate that the problem has been resolved.

Time Estimate (optional):
< 3h

Additional context (optional)
Especially needed for the control plane secret. Borrow documentation from https://cloud.google.com/config-connector/docs/how-to/managing-multiple-projects

[github-actions]

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