docs: fix Helm chart repository URL

Author: mahmut-Abi

README.md

@@ -47,7 +47,7 @@ The control plane is designed to stay Kubernetes-native. Gateway API resources r
 Helm is the recommended production installation path:
 
 ```bash
-helm repo add nantian-gw https://nantian-gw.github.io/helm-charts
+helm repo add nantian-gw https://charts.nantian.dev
 helm install nantian-gw nantian-gw/nantian-gw \
   --namespace nantian-gw \
   --create-namespace

README.zh-CN.md

@@ -89,7 +89,7 @@ spec:
 ### Helm（推荐用于生产环境）
 
 ```bash
-helm repo add nantian-gw https://nantian-gw.github.io/helm-charts
+helm repo add nantian-gw https://charts.nantian.dev
 helm install nantian-gw nantian-gw/nantian-gw \
   --namespace nantian-gw \
   --create-namespace