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

For package opts in to batch release. the repo checks needs to check the pr

For PR against main branch, the check follow the step

1. has an pending_changelogs folder
2. if has label post-release-<package name>, proceed with the existing check and END here.
3. Changelog.md and version in pubspec.yaml must not change
4. must have modification to any of the files in pending_changelogs folder includeing add/remove/modify, unless override: no change long needed and override: no version is needed label are used
5. All files inpending_changelogs must be formatted correctly

For PR against release branch

1. Use existing check for changelog and pubspec.yaml.