main doc: flutter.dev/go/batch-release-one-pager

For package opts in to batch release. The repo checks needs to

1. has an unreleased folder
2. if has label release-<package name>, proceed with the existing check end end here.
3. If has label override skip changelog and override skip version, do nothing and end here.
4. Has modification to files in unreleased folder, either remove, add, or modify existing.