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 <[email protected]>
PR-URL: #57527
Reviewed-By: Michaël Zasso <[email protected]>
Reviewed-By: Antoine du Hamel <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: LiviaMedeiros <[email protected]>
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: Chengzhong Wu <[email protected]>
Reviewed-By: Chemi Atlow <[email protected]>
Reviewed-By: Jake Yuesong Li <[email protected]>
Reviewed-By: Yagiz Nizipli <[email protected]>
Reviewed-By: Rafael Gonzaga <[email protected]>
Reviewed-By: Jordan Harband <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>
Reviewed-By: Ulises Gascón <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>
Reviewed-By: Xuguang Mei <[email protected]>

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: