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

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc