Add webhook installation to docs (nginx#505)

Previously, the installation docs didn't install the Gateway API
validating webhook. This comment adds it.

Fixes nginx#403

Author: miledxz

docs/installation.md

@@ -17,10 +17,10 @@ This guide walks you through how to install NGINX Kubernetes Gateway on a generi
    cd nginx-kubernetes-gateway
    ```
 
-1. Install the Gateway CRDs:
+1. Install the Gateway API resources from the standard channel (the CRDs and the validating webhook):
 
    ```
-   kubectl apply -k "github.com/kubernetes-sigs/gateway-api/config/crd?ref=v0.6.2"
+   kubectl apply -f https://github.com/kubernetes-sigs/gateway-api/releases/download/v0.6.2/standard-install.yaml
    ```
 
 1. Create the nginx-gateway Namespace: