docs for response override

Signed-off-by: Huabing Zhao <zhaohuabing@gmail.com>

site/content/en/latest/tasks/traffic/response-override.md

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+---
+title: "Response Override"
+---
+
+Response Override allows you to override the response from the backend with a custom one. This can be useful for scenarios such as returning a custom 404 page when the requested resource is not found or a custom 500 error message when the backend is failing.
+
+## Installation
+
+Follow the steps from the [Quickstart](../../quickstart) to install Envoy Gateway and the example manifest.
+Before proceeding, you should be able to query the example backend using HTTP.
+
+## Testing Response Override
+
+{{< tabpane text=true >}}
+{{% tab header="Apply from stdin" %}}
+
+```shell
+cat <<EOF | kubectl apply -f -
+apiVersion: gateway.envoyproxy.io/v1alpha1
+kind: BackendTrafficPolicy
+metadata:
+  name: response-override
+spec:
+  targetRef:
+    group: gateway.networking.k8s.io
+    kind: HTTPRoute
+    name: backend
+  responseOverride:
+    - match:
+        statusCodes:
+          - type: Value
+            value: 404
+      response:
+        contentType: text/plain
+        body:
+          type: Inline
+          inline: "Oops! Your request is not found."
+    - match:
+        statusCodes:
+          - type: Value
+            value: 500
+          - type: Range
+            range:
+              start: 501
+              end: 511
+      response:
+        contentType: application/json
+        body:
+          type: ValueRef
+          valueRef:
+            group: ""
+            kind: ConfigMap
+            name: response-override-config
+---
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: response-override-config
+data:
+  response.body: '{"error": "Internal Server Error"}'
+EOF
+```
+
+{{% /tab %}}
+{{% tab header="Apply from file" %}}
+Save and apply the following resource to your cluster:
+
+```yaml
+---
+apiVersion: gateway.envoyproxy.io/v1alpha1
+kind: BackendTrafficPolicy
+metadata:
+  name: response-override
+spec:
+  targetRef:
+    group: gateway.networking.k8s.io
+    kind: HTTPRoute
+    name: backend
+  responseOverride:
+    - match:
+        statusCodes:
+          - type: Value
+            value: 404
+      response:
+        contentType: text/plain
+        body:
+          type: Inline
+          inline: "Oops! Your request is not found."
+    - match:
+        statusCodes:
+          - type: Value
+            value: 500
+          - type: Range
+            range:
+              start: 501
+              end: 511
+      response:
+        contentType: application/json
+        body:
+          type: ValueRef
+          valueRef:
+            group: ""
+            kind: ConfigMap
+            name: response-override-config
+---
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: response-override-config
+data:
+  response.body: '{"error": "Internal Server Error"}'
+```
+{{% /tab %}}
+{{< /tabpane >}}
+```shell
+curl --verbose --header "Host: www.example.com" http://$GATEWAY_HOST/status/404
+```
+```console
+*   Trying 127.0.0.1:80...
+* Connected to 172.18.0.200 (172.18.0.200) port 80
+> GET /status/404 HTTP/1.1
+> Host: www.example.com
+> User-Agent: curl/8.5.0
+> Accept: */*
+>
+< HTTP/1.1 404 Not Found
+< content-type: text/plain
+< content-length: 32
+< date: Thu, 07 Nov 2024 09:22:29 GMT
+<
+* Connection #0 to host 172.18.0.200 left intact
+Oops! Your request is not found.
+```
+
+```shell
+curl --verbose --header "Host: www.example.com" http://$GATEWAY_HOST/status/500
+```
+
+```console
+*   Trying 127.0.0.1:80...
+* Connected to 172.18.0.200 (172.18.0.200) port 80
+> GET /status/500 HTTP/1.1
+> Host: www.example.com
+> User-Agent: curl/8.5.0
+> Accept: */*
+>
+< HTTP/1.1 500 Internal Server Error
+< content-type: application/json
+< content-length: 34
+< date: Thu, 07 Nov 2024 09:23:02 GMT
+<
+* Connection #0 to host 172.18.0.200 left intact
+{"error": "Internal Server Error"}
+```