Document GCS bucket retention policy impact on ClickHouse storage

content/en/altinity-kb-integrations/altinity-kb-google-s3-gcs.md を読み、既存の GCS セットアップ手順を確認します。保持ポリシーによって、merge、mutation、TTL の期限切れの際に ClickHouse による削除がどのように妨げられるかを説明する警告を追加し、保持ポリシーのない bucket を使用することを推奨します。ストレージの増加と運用コストのリスクが明確に文書化されていれば完了です。

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.