doc: onboarding process extras

Callout some practices explicitly, so that
the process is followed in a similar manner

PR-URL: https://github.com/nodejs/node/pull/34455


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Andrey Pechkurov <apechkurov@gmail.com>