doc: fix broken link in COLLABORATOR_GUIDE.md
Checklist
-
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc
Description of change
The link to the onboarding-extras.md document is broken and can not be called in COLLABORATOR_GUIDE.md as it is now located in the folder docs. I've changed the link that it points to the document.