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/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:headlamp:headlamp
 ```
 
 ### 5. Network Policies