doc: recommend watching the collaborators repo in the onboarding doc

It might be easy to miss this repo. For example, I forgot rewatching
this repo when I rejoined. So it would be a good idea to mention this in
the collaborator onboarding guide.

Signed-off-by: Darshan Sen <raisinten@gmail.com>
PR-URL: #57527
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: LiviaMedeiros <livia@cirno.name>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Chemi Atlow <chemi@atlow.co.il>
Reviewed-By: Jake Yuesong Li <jake.yuesong@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Jordan Harband <ljharb@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Xuguang Mei <meixuguang@gmail.com>

Author: RaisinTen

onboarding.md

@@ -53,6 +53,8 @@ onboarding session.
     set up email
   * Watching the main repository will flood your inbox (several hundred
     notifications on typical weekdays), so be prepared
+  * Watching the discussions in the
+    [collaborators repo](https://github.com/nodejs/collaborators) is recommended.
 
 The project has a venue for real-time discussion: