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/troubleshooting/metrics.md

@@ -47,7 +47,7 @@ This requires `get` on `pods/proxy` in `kube-system`:
 ```bash
 kubectl auth can-i get pods/proxy \
   -n kube-system \
-  --as=system:serviceaccount:kube-system:headlamp
+  --as=system:serviceaccount:<your-namespace>:headlamp
 ```
 
 ### 5. Network Policies