add: documentation

docs/20_discovery/s3-service-info.md

@@ -0,0 +1,18 @@
+# S3 Service Discovery
+
+**Date:** 2026-01-28
+**Source:** User input
+
+## Documentation
+
+*   **Description**: https://docs.s3.msk-1.ngcloud.ru/s3/s3.description.html
+
+## Key Concepts
+
+*   **S3 Instance (Account/User)**: This is the service level entity. It represents the S3 account/user credentials (access_key, secret_key). 
+    *   *Usage*: PostgreSQL dependencies refer to this **S3 Instance**, NOT a specific bucket.
+    *   Examples of UIDs: `xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx`.
+*   **S3 Bucket**: A container for objects within an S3 Instance.
+
+## Future Implementation Notes
+When implementing or documenting `nubes_s3_instance` (or similar), refer to the provided documentation link.