doc: simplify first-time contributors section of Collaborator Guide

PR-URL: https://github.com/nodejs/node/pull/24387


Reviewed-By: Ouyang Yadong <oyydoibh@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>