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/deployment/helm.md

@@ -9,7 +9,7 @@ helm repo add headlamp https://headlamp-k8s.github.io/headlamp/
 helm repo update
 
 helm install headlamp headlamp/headlamp \
-  --namespace kube-system \
+  --namespace <your-namespace> \
   --create-namespace \
   --set config.pluginsDir=/headlamp/plugins \
   --set pluginsManager.sources[0].name=tns-csi \
@@ -44,7 +44,7 @@ Apply:
 
 ```bash
 helm install headlamp headlamp/headlamp \
-  --namespace kube-system \
+  --namespace <your-namespace> \
   -f headlamp-values.yaml
 ```
 
@@ -55,7 +55,7 @@ apiVersion: source.toolkit.fluxcd.io/v1
 kind: HelmRepository
 metadata:
   name: headlamp
-  namespace: kube-system
+  namespace: <your-namespace>
 spec:
   interval: 12h
   url: https://headlamp-k8s.github.io/headlamp/
@@ -64,7 +64,7 @@ apiVersion: helm.toolkit.fluxcd.io/v2
 kind: HelmRelease
 metadata:
   name: headlamp
-  namespace: kube-system
+  namespace: <your-namespace>
 spec:
   interval: 1h
   chart:
@@ -74,7 +74,7 @@ spec:
       sourceRef:
         kind: HelmRepository
         name: headlamp
-        namespace: kube-system
+        namespace: <your-namespace>
   values:
     config:
       pluginsDir: /headlamp/plugins
@@ -122,7 +122,7 @@ metadata:
 subjects:
   - kind: ServiceAccount
     name: headlamp
-    namespace: kube-system
+    namespace: <your-namespace>
 roleRef:
   kind: ClusterRole
   name: headlamp-tns-csi-reader
@@ -136,7 +136,7 @@ To upgrade to a new plugin version, update the `url` in your values and apply:
 
 ```bash
 helm upgrade headlamp headlamp/headlamp \
-  --namespace kube-system \
+  --namespace <your-namespace> \
   -f headlamp-values.yaml
 ```