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

Users choose their own namespace for Headlamp. Replace all hardcoded
namespace references (headlamp, kube-system) in user-facing docs with
<your-namespace> so users substitute their own value.

Conventions:
- Helm install: --namespace <your-namespace> --create-namespace
- kubectl commands: -n <your-namespace>
- YAML metadata: namespace: <your-namespace>
- Prose: "the namespace where Headlamp is installed"

Out-of-scope references left untouched:
- kube-system in NetworkPolicy selectors (API server namespace)
- polaris namespace references (upstream workload namespace)
- Source code and test files

Refs: PRI-433

Co-Authored-By: Paperclip <noreply@paperclip.ing>

docs/user-guide/rbac-permissions.md

@@ -65,7 +65,7 @@ metadata:
 subjects:
   - kind: ServiceAccount
     name: headlamp # Adjust to your Headlamp SA name
-    namespace: headlamp # Adjust to Headlamp's namespace
+    namespace: <your-namespace>
 roleRef:
   kind: Role
   name: polaris-proxy-reader
@@ -75,7 +75,7 @@ roleRef:
 **Adjust for your environment:**
 
 - `subjects[0].name` - Your Headlamp service account name (often `headlamp`)
-- `subjects[0].namespace` - Namespace where Headlamp runs (often `headlamp`)
+- `subjects[0].namespace` - Namespace where Headlamp is installed
 
 ### Step 3: Apply and Verify
 
@@ -267,7 +267,7 @@ metadata:
 subjects:
   - kind: ServiceAccount
     name: headlamp
-    namespace: headlamp
+    namespace: <your-namespace>
 roleRef:
   kind: Role
   name: polaris-proxy-reader
@@ -281,7 +281,7 @@ metadata:
 subjects:
   - kind: ServiceAccount
     name: headlamp
-    namespace: headlamp
+    namespace: <your-namespace>
 roleRef:
   kind: Role
   name: polaris-proxy-reader