doc: simplify COLLABORATOR_GUIDE.md instructions

Remove bulleted list formatting. Only check email/name for new
contributors. Change `full name` to `name` and remove italics to avoid
incorrect implication that we need a "legal name".

PR-URL: https://github.com/nodejs/node/pull/19458


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>