doc: document the collaborator nomination process
Also describes the privileges of the collaborators and improve the instructions of the onboarding PR.
This is a draft done after the discussion in https://github.com/nodejs/node/issues/18090 and this week's TSC meeting, any feedback is welcome!
Refs: https://github.com/nodejs/TSC/pull/472 Fixes: https://github.com/nodejs/node/issues/18090
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc