doc: move Code of Conduct to admin repo

The CoC and moderation guidelines now live in the Admin repo.

This PR updates the copy of CODE_OF_CONDUCT.md and references in
other documents to the proper location.

PR-URL: https://github.com/nodejs/node/pull/17301


Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>