fix: update Headlamp Helm repo URL from headlamp-k8s to kubernetes-sigs

The Headlamp project moved to the kubernetes-sigs org. The old Helm chart
repository URL (headlamp-k8s.github.io) returns 404, causing E2E workflow
failure at the `helm repo add` step.

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

docs/deployment/helm.md

@@ -19,7 +19,7 @@ Helm provides the easiest way to deploy and manage the plugin in production. Thi
 
 ```bash
 # Add Headlamp Helm repository
-helm repo add headlamp https://headlamp-k8s.github.io/headlamp/
+helm repo add headlamp https://kubernetes-sigs.github.io/headlamp/
 helm repo update
 ```
 
@@ -210,7 +210,7 @@ metadata:
   namespace: flux-system
 spec:
   interval: 1h
-  url: https://headlamp-k8s.github.io/headlamp/
+  url: https://kubernetes-sigs.github.io/headlamp/
 ```
 
 ### HelmRelease

docs/getting-started/prerequisites.md

@@ -84,7 +84,7 @@ kubectl -n kube-system get deployment headlamp -o jsonpath='{.spec.template.spec
 
 ```bash
 # Add Headlamp Helm repository
-helm repo add headlamp https://headlamp-k8s.github.io/headlamp/
+helm repo add headlamp https://kubernetes-sigs.github.io/headlamp/
 helm repo update
 
 # Install Headlamp