Docs: add basic grpc example (nginxinc#1905)

* Add basic example

* Change note wording and code block white spaces

* Change grpc app tag

examples/grpc-routing/README.md

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+# gRPC Routing Example
+
+In this example, we deploy NGINX Gateway Fabric, a simple gRPC web application, and then configure NGINX Gateway Fabric
+to route traffic to that application using GRPCRoute resources.
+
+## Running the Example
+
+## 1. Deploy NGINX Gateway Fabric
+
+1. Follow the [installation instructions](https://docs.nginx.com/nginx-gateway-fabric/installation/) to deploy NGINX Gateway Fabric.
+    > **Important**: Ensure the Gateway APIs from the experimental channel are installed and that NGF is deployed with the Gateway experimental features enabled.
+
+1. Save the public IP address of NGINX Gateway Fabric into a shell variable:
+
+    ```text
+    GW_IP=XXX.YYY.ZZZ.III
+    ```
+
+1. Save the port of NGINX Gateway Fabric:
+
+    ```text
+    GW_PORT=<port number>
+    ```
+
+## 2. Deploy the Helloworld Application
+
+1. Create the two helloworld Deployments and Services:
+
+    ```shell
+    kubectl apply -f helloworld.yaml
+    ```
+
+1. Check that the Pods are running in the `default` Namespace:
+
+    ```shell
+    kubectl -n default get pods
+    ```
+
+    ```text
+    NAME                      READY   STATUS    RESTARTS   AGE
+    grpc-infra-backend-v1-766c7d6788-rg92p   1/1     Running   0          12s
+    grpc-infra-backend-v2-546f7c8d48-mjkkx   1/1     Running   0          12s
+    ```
+
+   Save these pod names into variables:
+
+    ```text
+    POD_V1=<grpc-infra-backend-v1-xxxxxxxxxx-xxxxx>
+    POD_V2=<grpc-infra-backend-v2-xxxxxxxxxx-xxxxx>
+    ```
+
+## 3. Configure Routing
+
+There are 3 options to configure gRPC routing. To access the application and test the routing rules, we will use [grpcurl](https://github.com/fullstorydev/grpcurl?tab=readme-ov-file#installation).
+
+### 3a. Configure exact method matching based routing
+
+1. Create the Gateway and GRPCRoute resources:
+
+    ```shell
+    kubectl apply -f exact-method.yaml
+    ```
+
+2. Test the Application:
+
+    ```shell
+    grpcurl -plaintext -proto grpc.proto -authority bar.com -d '{"name": "exact"}' ${GW_IP}:${GW_PORT} helloworld.Greeter/SayHello
+    ```
+
+    ```text
+    {
+        "message": "Hello exact"
+    }
+    ```
+
+3. Clean up the Gateway and GRPCRoute resources:
+
+    ```shell
+    kubectl delete -f exact-method.yaml
+    ```
+
+### 3b. Configure hostname based routing
+
+1. Create the Gateway and GRPCRoute resources:
+
+    ```shell
+    kubectl apply -f hostname.yaml
+    ```
+
+2. Test the Application:
+
+    ```shell
+    grpcurl -plaintext -proto grpc.proto -authority bar.com -d '{"name": "bar server"}' ${GW_IP}:${GW_PORT} helloworld.Greeter/SayHello
+    ```
+
+    ```text
+    {
+        "message": "Hello bar server"
+    }
+    ```
+
+   To make sure this came from the correct server, we can check the application server logs:
+
+    ```shell
+    kubectl logs ${POD_V1}
+    ```
+
+    ```text
+    2024/04/29 09:26:54 server listening at [::]:50051
+    2024/04/29 09:28:54 Received: bar server
+    ```
+
+   Now we'll send a request to `foo.bar.com`
+
+    ```shell
+    grpcurl -plaintext -proto grpc.proto -authority foo.bar.com -d '{"name": "foo bar server"}' ${GW_IP}:${GW_PORT} helloworld.Greeter/SayHello
+    ```
+
+    ```text
+    {
+        "message": "Hello foo bar server"
+    }
+    ```
+
+   This time, we'll check the POD_V2 logs:
+
+    ```shell
+    kubectl logs ${POD_V2}
+    ```
+
+    ```text
+    2024/04/29 09:26:55 server listening at [::]:50051
+    2024/04/29 09:29:46 Received: foo bar server
+    ```
+
+3. Clean up the Gateway and GRPCRoute resources:
+
+    ```shell
+    kubectl delete -f hostname.yaml
+    ```
+
+### 3c. Configure headers based routing
+
+1. Create the Gateway and GRPCRoute resources:
+
+    ```shell
+    kubectl apply -f headers.yaml
+    ```
+
+2. Test the Application:
+
+    ```shell
+    grpcurl -plaintext -proto grpc.proto -authority bar.com -d '{"name": "version one"}' -H 'version: one' ${GW_IP}:${GW_PORT} helloworld.Greeter/SayHello
+    ```
+
+    ```text
+    {
+        "message": "Hello version one"
+    }
+    ```
+
+   To make sure this came from the correct server, we can check the application server logs:
+
+    ```shell
+    kubectl logs ${POD_V1}
+    ```
+
+    ```text
+    <...>
+    2024/04/29 09:30:27 Received: version one
+    ```
+
+   Now we'll send a request with the header `version: two`
+
+    ```shell
+    grpcurl -plaintext -proto grpc.proto -authority bar.com -d '{"name": "version two"}' -H 'version: two' ${GW_IP}:${GW_PORT} helloworld.Greeter/SayHello
+    ```
+
+    ```text
+    {
+        "message": "Hello version two"
+    }
+    ```
+
+   This time, we'll check the POD_V2 logs:
+
+    ```shell
+    kubectl logs ${POD_V2}
+    ```
+
+    ```text
+    <...>
+    2024/04/29 09:32:46 Received: version two
+    ```
+
+   Finally, we'll send a request with the headers `version: two` and `color: orange`
+
+    ```shell
+    grpcurl -plaintext -proto grpc.proto -authority bar.com -d '{"name": "version two orange"}' -H 'version: two' -H 'color: orange' ${GW_IP}:${GW_PORT} helloworld.Greeter/SayHello
+    ```
+
+   ```text
+   {
+     "message": "Hello version two orange"
+   }
+   ```
+
+   Now check the POD_V1 logs again:
+
+   ```shell
+   kubectl logs ${POD_V1}
+   ```
+
+   ```text
+   <...>
+   2024/04/29 09:30:27 Received: version one
+   2024/04/29 09:33:26 Received: version two orange
+   ```
+
+3. Clean up the Gateway and GRPCRoute resources:
+
+   ```shell
+   kubectl delete -f headers.yaml
+   ```

examples/grpc-routing/exact-method.yaml

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+apiVersion: gateway.networking.k8s.io/v1beta1
+kind: Gateway
+metadata:
+  name: same-namespace
+spec:
+  gatewayClassName: nginx
+  listeners:
+  - name: http
+    port: 80
+    protocol: HTTP
+    allowedRoutes:
+      namespaces:
+        from: Same
+---
+apiVersion: gateway.networking.k8s.io/v1alpha2
+kind: GRPCRoute
+metadata:
+  name: exact-matching
+spec:
+  parentRefs:
+  - name: same-namespace
+  rules:
+  - matches:
+    - method:
+        service: helloworld.Greeter
+        method: SayHello
+    backendRefs:
+    - name: grpc-infra-backend-v1
+      port: 8080

examples/grpc-routing/grpc.proto

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+// Copyright 2015 gRPC authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto3";
+
+option go_package = "google.golang.org/grpc/examples/helloworld/helloworld";
+option java_multiple_files = true;
+option java_package = "io.grpc.examples.helloworld";
+option java_outer_classname = "HelloWorldProto";
+
+package helloworld;
+
+// The greeting service definition.
+service Greeter {
+  // Sends a greeting
+  rpc SayHello (HelloRequest) returns (HelloReply) {}
+}
+
+// The request message containing the user's name.
+message HelloRequest {
+  string name = 1;
+}
+
+// The response message containing the greetings
+message HelloReply {
+  string message = 1;
+}

examples/grpc-routing/headers.yaml

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+apiVersion: gateway.networking.k8s.io/v1beta1
+kind: Gateway
+metadata:
+  name: same-namespace
+spec:
+  gatewayClassName: nginx
+  listeners:
+  - name: http
+    port: 80
+    protocol: HTTP
+    allowedRoutes:
+      namespaces:
+        from: Same
+---
+apiVersion: gateway.networking.k8s.io/v1alpha2
+kind: GRPCRoute
+metadata:
+  name: grpc-header-matching
+spec:
+  parentRefs:
+  - name: same-namespace
+  rules:
+  # Matches "version: one"
+  - matches:
+    - headers:
+      - name: version
+        value: one
+    backendRefs:
+    - name: grpc-infra-backend-v1
+      port: 8080
+  # Matches "version: two"
+  - matches:
+    - headers:
+      - name: version
+        value: two
+    backendRefs:
+    - name: grpc-infra-backend-v2
+      port: 8080
+  # Matches "version: two" AND "color: orange"
+  - matches:
+    - headers:
+      - name: version
+        value: two
+      - name: color
+        value: orange
+    backendRefs:
+    - name: grpc-infra-backend-v1
+      port: 8080
+  # Matches "color: blue" OR "color: green"
+  - matches:
+    - headers:
+      - name: color
+        value: blue
+    - headers:
+      - name: color
+        value: green
+    backendRefs:
+    - name: grpc-infra-backend-v1
+      port: 8080
+  # Matches "color: red" OR "color: yellow"
+  - matches:
+    - headers:
+      - name: color
+        value: red
+    - headers:
+      - name: color
+        value: yellow
+    backendRefs:
+    - name: grpc-infra-backend-v2
+      port: 8080