doc: reorganize collaborator guide

* Add sections about first time contributions, code reviews
  and seeking consensus, waiting for approvals, testing and CI
* Move paragraphs to more suitable sections
* Update table of contents
* Document the fast-tracking process

PR-URL: https://github.com/nodejs/node/pull/17056


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>