doc: split COLLABORATOR_GUIDE.md

I was going through COLLABORATOR_GUIDE.md as part of my onboarding as a NodeJS Collaborator (#19067 (closed)) Noticed that with 783 lines it's pretty long to read and can be split.

I followed PR of @joyeecheung which had split CONTRIBUTING.md in #18271 as a reference

The edits in COLLABORATOR_GUIDE.md in this PR can be viewed in my private branch at https://github.com/trivikr/node/blob/split-collab-guide/COLLABORATOR_GUIDE.md

cc @nodejs/documentation

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc