doc: add offboarding doc

Add minimal document for what to do when a Collaborator moves to
Collaborator Emeritus or leaves the project.

PR-URL: https://github.com/nodejs/node/pull/21103


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Lance Ball <lball@redhat.com>
Reviewed-By: Matheus Marchini <matheus@sthima.com>