doc: add offboarding doc
Add minimal document for what to do when a Collaborator moves to Collaborator Emeritus or leaves the project.
Checklist
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documentation is changed or added -
commit message follows commit guidelines
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Add minimal document for what to do when a Collaborator moves to Collaborator Emeritus or leaves the project.