doc: add GitHub email set up link to COLLABORATOR_GUIDE

PR-URL: https://github.com/nodejs/node/pull/22525


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: George Adams <george.adams@uk.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>