doc: clarify part of onboarding guide regarding adding to teams

There are a number of teams (for example, the build team) which require
more than just showing interest as a prerequisite for being added, so
this change adds a link to the set of subsystem teams where new
Collaborators can be added directy.

Also, the doc didn't actually mention if the new Collaborator should be
added to the team. This change also clarifies that.

Refs: nodejs/build#3003
Signed-off-by: Darshan Sen <raisinten@gmail.com>

PR-URL: nodejs/node#44024
Reviewed-By: Feng Yu <F3n67u@outlook.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

onboarding.md

@@ -19,8 +19,8 @@ onboarding session.
 
 * Prior to the onboarding session, add the new Collaborator to
   [the collaborators team](https://github.com/orgs/nodejs/teams/collaborators).
-* Ask them if they want to join any subsystem teams. See
-  [Who to CC in the issue tracker][who-to-cc].
+* Ask them if they want to join any [subsystem teams](https://github.com/orgs/nodejs/teams/core/teams)
+  and add them accordingly. See [Who to CC in the issue tracker][who-to-cc].
 
 ## Onboarding session