doc: add author-ready label ref to onboarding doc

The "Labels" section of doc/guides/onboarding-extras.md was missing a
reference to the `author-ready` label. This commit adds a description
similar to the definition found in doc/guides/collaborator-guide.md
with the goal of making it easier for new contributors to find labels
info all in one place.

PR-URL: https://github.com/nodejs/node/pull/34381


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Juan José Arboleda <soyjuanarbol@gmail.com>
Reviewed-By: Gerhard Stöbich <deb2001-github@yahoo.de>
Reviewed-By: Myles Borins <myles.borins@gmail.com>