doc: update collaborator guide

This also updates some parts of the onboarding. It is mainly about
how to handle pull requests, how and when to start a CI, when to add
the `ready` label and some other minor changes.

PR-URL: https://github.com/nodejs/node/pull/19116


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Chen Gang <gangc.cxy@foxmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Glen Keane <glenkeane.94@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>