Create an onboarding and offboarding document

[bjohansebas]

It would be good to create a document for when someone leaves the organization, especially for the TC and for those groups that have access to private parts of the project (like the security triage Slack).

Also, it would be useful to have one for when someone joins a group like the security team, the performance team, or the TC itself.

This would be related to #324.