docs: document version error detection and handling (webhook rejections + ExtensionRollback event)

[xgerman]

Summary

Version handling is surfaced in DocumentDB.status (schemaVersion, documentDBImage, gatewayImage) and the upgrade flow enforces version-safety rules, but the documentation does not explain how to detect and handle version error conditions.

Undocumented error signals

The code produces version-related error signals that are not documented anywhere under docs/operator-public-documentation/preview/:

1. Webhook rejection on image rollback (operator/src/internal/webhook/documentdb_webhook.go): setting documentDBVersion/image below the installed schema fails kubectl apply synchronously with:

   image rollback blocked: requested version X is older than installed schema version Y. ALTER EXTENSION has no downgrade path...

2. Webhook rejection when schemaVersion exceeds binary:

   schemaVersion X exceeds the binary version Y; schema version must be <= binary version

3. ExtensionRollback warning Event emitted by the controller (operator/src/internal/controller/documentdb_controller.go) as a defense-in-depth guard — detectable via kubectl describe documentdb / kubectl get events.

The operations/upgrades.md "Rollback and Recovery" section explains the rules ("the operator rejects it") but never shows the actual error text or tells users how to detect these conditions (events / describe / apply failure) and what to do for each.

Ask

Add a "Troubleshooting version errors" section (in upgrades.md or a new page) covering, for each signal above: how to detect it, the exact message, and the recovery action.

Acceptance criteria

• Troubleshooting section documents the two webhook rejections + the ExtensionRollback event.
• Each includes detection steps (apply failure / kubectl get events / describe) and a concrete recovery action.