docs(observability): add comment to explain annotation

Author: Bibas

observability/cockpit/how-to/send-log-from-k8s-to-cockpit.mdx

@@ -60,6 +60,7 @@ logs:
   pod_logs:
     enabled: true
     discovery: "all"
+    # This annotation is mandatory to gather your logs
     annotation: "cockpit.scw.cloud/logs.autogather"
     # -- Only capture logs from pods in these namespaces (`[]` means all namespaces).
     namespaces: []