doc: remove onboarding-extras

Migrate last bit of onboarding-extras content to collaborator-guide and
remove onboarding-extras.

PR-URL: https://github.com/nodejs/node/pull/39252


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Richard Lau <rlau@redhat.com>
Reviewed-By: Beth Griggs <bgriggs@redhat.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>