Document GCS bucket retention policy impact on ClickHouse storage

Leggi content/en/altinity-kb-integrations/altinity-kb-google-s3-gcs.md e verifica le istruzioni esistenti per la configurazione di GCS. Aggiungi un avviso che spieghi come le policy di retention impediscano a ClickHouse di eliminare i dati durante merge, mutazioni e scadenza del TTL, e raccomanda di utilizzare un bucket senza una policy di retention. Il lavoro è completato quando il rischio di crescita dello storage e dei costi operativi è documentato chiaramente.

Problem

The current GCS documentation (content/en/altinity-kb-integrations/altinity-kb-google-s3-gcs.md) only covers basic setup (HMAC keys, endpoint configuration) but doesn't mention an important operational caveat.

Missing Information

When a GCS bucket has a retention policy enabled, objects cannot be deleted until the retention period expires. This causes issues with ClickHouse because it regularly deletes data during:

• Part merges - old parts are removed after merge
• Mutations - replaced data is deleted
• TTL expiration - expired data is purged

If the bucket retention policy prevents deletion, old data accumulates and storage grows exponentially, leading to unexpected costs and potential operational issues.

Suggested Addition

Add a warning/note to the GCS documentation explaining:

1. Bucket retention policies prevent ClickHouse from deleting obsolete data
2. This causes continuous storage growth
3. Recommendation to disable retention policy or use a bucket without one for ClickHouse storage

If this seems like a good enhancement, I'd be happy to take this up and submit a PR.