doc: reorganize COLLABORATOR_GUIDE.md

Based on feedback during a recent collaborator onboarding, move the
metadata description closer to where the instructions for editing a
commit message are.

Indicate which metadata are required and which are optional.

Make the punctuation more consistent. Previously, sentences prior to
instructions ended in a period, a colon, or no punctuation at all. Colon
seems best, so changed the Technical How-To section to use colons.

PR-URL: https://github.com/nodejs/node/pull/15710


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Benedikt Meurer <benedikt.meurer@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>