doc: improve best practices in onboarding-extras

PR-URL: https://github.com/nodejs/node/pull/19315


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>