Document resources staging groups and finalization

Includes instructions on how to expose new public resource IDs
and how to "remove" unused resource IDs before finalization.

Temporarily restricts OWNERS to ensure T remains stable until
a long term static analysis solution is found.

Fixes the script for finalization to handle split files and
seed the next release's staging-public-group blocks.

Bug: 222537368

Test: manual, checked build before and after mock finalization

Change-Id: I901a80e487f5eaab9689f469196648535857e0e1