Define API repository setup for Release Automation

[hdamker]

Problem description

API repositories that want to use the Release Automation need specific configuration (rulesets, caller workflow, CODEOWNERS adjustments, CHANGELOG directory structure). This setup needs to be documented so it can be applied consistently across all repositories.

Proposed solution

Create a repository setup guide (release_automation/docs/repository-setup.md) that documents:

1. Repository rulesets: Branch protection for release-snapshot/** branches with GitHub App bypass
2. Caller workflow: .github/workflows/release-automation.yml that references the reusable workflow in tooling
3. CHANGELOG directory structure: Migration from single CHANGELOG.md to CHANGELOG/ directory with per-release files
4. Verification checklist: Steps to confirm a repository is correctly configured

Additional context

An initial version of repository-setup.md exists on the release-automation branch. It needs updating to reflect the GitHub App bypass approach and the consolidated single-ruleset design.

[hdamker]

Please review #75 with the updated description ... it isn't linked as PR for this issues, as the PR goes into the feature branch release-automation, not directly to main.