docs: replace hardcoded namespace with <your-namespace> placeholder

Users choose their own namespace for Headlamp. Replace all hardcoded
kube-system references that indicate Headlamp's install namespace with
<your-namespace> so users substitute their own value.

Upstream workload references left untouched:
- tns-csi controller pods in kube-system (upstream CSI driver)
- NetworkPolicy selectors targeting kube-system
- API server proxy paths to kube-system pods

Refs: PRI-434

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: <your-namespace>
 roleRef:
   kind: ClusterRole
   name: headlamp-tns-csi-reader
@@ -99,7 +99,7 @@ metadata:
 subjects:
   - kind: ServiceAccount
     name: headlamp
-    namespace: kube-system
+    namespace: <your-namespace>
 roleRef:
   kind: Role
   name: headlamp-tns-csi-benchmark