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/getting-started/installation.md

@@ -32,7 +32,7 @@ apiVersion: helm.toolkit.fluxcd.io/v2
 kind: HelmRelease
 metadata:
   name: headlamp
-  namespace: kube-system
+  namespace: <your-namespace>
 spec:
   chart:
     spec:

docs/getting-started/quick-start.md

@@ -34,7 +34,7 @@ pluginsManager:
 Then upgrade your Headlamp release:
 
 ```bash
-helm upgrade headlamp headlamp/headlamp -f values.yaml -n kube-system
+helm upgrade headlamp headlamp/headlamp -f values.yaml -n <your-namespace>
 ```
 
 ## Step 2: Configure RBAC
@@ -70,7 +70,7 @@ metadata:
 subjects:
   - kind: ServiceAccount
     name: headlamp
-    namespace: kube-system
+    namespace: <your-namespace>
 roleRef:
   kind: ClusterRole
   name: headlamp-tns-csi-reader
@@ -78,7 +78,7 @@ roleRef:
 EOF
 ```
 
-Adjust `name: headlamp` and `namespace: kube-system` to match your Headlamp service account.
+Adjust `name: headlamp` and `namespace: <your-namespace>` to match your Headlamp service account.
 
 ## Step 3: Verify