doc: move who-to-cc to COLABORATOR_GUIDE.md

Put the "who to cc" information in the COLLABORATOR_GUIDE. The
onboarding-extras doc is a bit of miscellaneous collection. Rather than
stashing things in a junk drawer doc, put them where they are relevant.

PR-URL: https://github.com/nodejs/node/pull/19460


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>