[workflows] Add a new workflow for checking commit access qualifications (#93301)

This workflow produces a list of users that should be moved to the LLVM
Triagers team. An admin will review the list and then manually move
people to the LLVM Triagers Team.

I've also added some documentation to the Developer Policy about the
Triage Role.

See
https://discourse.llvm.org/t/rfc2-new-criteria-for-commit-access/77110
for more details.

Author: tstellar