doc: add refack to collaborators

PR-URL: https://github.com/nodejs/node/pull/12277
Refs: https://github.com/nodejs/node/blob/master/doc/onboarding.md


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>