doc: improve onboarding instructions

- Suggest to use a TOTP app if the new collaborator cannot receive
  SMS messages from GitHub
- Suggest to install node-core-utils and set up credentials before
  the onboarding session.
- Ask about subsystem teams that they want to join
- Note about squashing commits
- Other small fixes

PR-URL: https://github.com/nodejs/node/pull/19108


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>