Provide example policy for S3 storage. (#7407)

**What this PR does / why we need it**:
This should make it easier for users to set the proper policies for
their S3 storage.

**Which issue(s) this PR fixes**:
Relates to #7403

**Special notes for your reviewer**:

**Checklist**
- [ ] Reviewed the `CONTRIBUTING.md` guide
- [x] Documentation added
- [ ] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/upgrading/_index.md`

docs/sources/operations/storage/_index.md

@@ -58,6 +58,35 @@ When using S3 as object storage, the following permissions are needed:
 
 Resources: `arn:aws:s3:::<bucket_name>`, `arn:aws:s3:::<bucket_name>/*`
 
+The following policy sets these permissions
+
+```json
+{
+    "Version": "2012-10-17",
+    "Statement": [
+        {
+            "Sid": "LokiStorage",
+            "Effect": "Allow",
+            "Principal": {
+                "AWS": [
+                    "arn:aws:iam::<account_ID>"
+                ]
+            },
+            "Action": [
+                "s3:ListBucket",
+                "s3:PutObject",
+                "s3:GetObject",
+                "s3:DeleteObject"
+            ],
+            "Resource": [
+                "arn:aws:s3:::<bucket_name>",
+                "arn:aws:s3:::<bucket_name>/*"
+            ]
+        }
+    ]
+}
+```
+
 ### DynamoDB
 
 When using DynamoDB for the index, the following permissions are needed: