docs: update Headlamp install namespace from kube-system to headlamp

Updates documentation to reflect that Headlamp is installed in the
'headlamp' namespace (not 'kube-system'). Only documentation files
that reference the Headlamp install namespace are changed.

Changed files:
- docs/deployment/production.md: NetworkPolicy namespaceSelector
- docs/troubleshooting/network-problems.md: NetworkPolicy namespaceSelector
- docs/user-guide/rbac-permissions.md: NetworkPolicy namespaceSelector
- e2e/README.md: kubectl commands for local E2E testing

Files NOT changed (upstream workload namespace - out of scope per PRI-340):
- Source files, tests, or configs referencing where Polaris runs

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

docs/user-guide/rbac-permissions.md

@@ -318,7 +318,7 @@ spec:
     - from:
         - namespaceSelector:
             matchLabels:
-              kubernetes.io/metadata.name: kube-system
+              kubernetes.io/metadata.name: headlamp
         - podSelector:
             matchLabels:
               component: kube-apiserver