docs: migrate Headlamp install namespace from kube-system to headlamp

Doc-only: redirect all references to Headlamp's own install
namespace from kube-system to headlamp, except:
- Driver namespace (CLAUDE.md) stays kube-system (upstream)
- CSI controller API paths (docs/architecture/overview.md) stay
  kube-system (upstream workload)

Co-Authored-By: Paperclip <noreply@paperclip.ing>

docs/user-guide/rbac.md

@@ -47,7 +47,7 @@ metadata:
 subjects:
   - kind: ServiceAccount
     name: headlamp          # adjust to your Headlamp service account name
-    namespace: kube-system  # adjust to your Headlamp namespace
+    namespace: headlamp  # adjust to your Headlamp namespace
 roleRef:
   kind: ClusterRole
   name: headlamp-tns-csi-reader
@@ -99,7 +99,7 @@ metadata:
 subjects:
   - kind: ServiceAccount
     name: headlamp
-    namespace: kube-system
+    namespace: headlamp
 roleRef:
   kind: Role
   name: headlamp-tns-csi-benchmark