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/getting-started/installation.md

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