Document GCS bucket retention policy impact on ClickHouse storage

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.

Read content/en/altinity-kb-integrations/altinity-kb-google-s3-gcs.md and review its existing GCS setup guidance. Add a warning explaining how retention policies block ClickHouse deletion during merges, mutations, and TTL expiration, and recommend using a bucket without a retention policy. Done means the storage-growth and operational-cost risk is clearly documented.