docs: document backup API endpoints based on code

[Copilot]

The backup API docs only covered StartBackup, RestoreBackup, and ListLocations. This PR fills in the remaining endpoints from backup.proto, locations.proto, restores.proto, and artifacts.proto.

New documentation files

• manage-locations.md — AddLocation, ChangeLocation, RemoveLocation, TestLocationConfig (filesystem & S3 examples, force-remove, config test)
• schedule-backup.md — ScheduleBackup, ListScheduledBackups, ChangeScheduledBackup, RemoveScheduledBackup (cron, mode descriptions, retention)
• list-artifacts.md — ListArtifacts, DeleteArtifact (with remove_files), ListArtifactCompatibleServices, ListPitrTimeranges, GetLogs (backup job)
• list-restores.md — ListRestores (with status values), GetLogs (restore job)

Updated

• overview.md — Restructured into sections (Backup Locations, On-Demand and Scheduled Backups, Backup Artifacts, Restore) linking to all pages.

• API Docs updated

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.

[it-percona-cla]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}