doc: revise code review guidelines

Revise the Code Reviews section of the Collaborator Guide to remove
redundant statements, simplify text and structure for easier
comprehension, and remove material that does not reflect current
practices.

PR-URL: https://github.com/nodejs/node/pull/24790


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>