Custom Controller Example Page on Agones Website (#3725)

* Custom Controller Example Page on Agones Website

examples/custom-controller/README.md

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+# Custom Controller for Agones Game Servers
+
+This is an example of how to write a customer controller integrated with Agones resources.
+
+This example shows how to create, deploy and run a Custom Kubernetes Controller for Agones that logs changes to GameServers and modifies their labels.
+
+For detailed instructions refer to the [Agones Documentation](https://agones.dev/site/docs/examples/custom-controller/).

site/content/en/docs/Examples/custom-controller.md

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+---
+title: "Custom Controller for Agones Game Servers"
+linkTitle: "Custom Controller"
+date:
+publishDate:
+description: >
+  This Custom Controller example shows how to create, deploy and run a Custom Kubernetes Controller for Agones that logs changes to GameServers and modifies their labels.
+---
+
+## Prerequisite
+
+To get started, ensure the following prerequisites are met:
+
+  - You have a running Kubernetes cluster.
+
+  - Agones is installed on your cluster. Refer [Agones guide](https://agones.dev/site/docs/installation/install-agones/).
+
+  - (Optional) Review {{< ghlink href="examples/custom-controller" >}}Custom Controller code{{< /ghlink >}} to see the details of this example.
+
+## Create a Custom Controller
+
+Let's create a custom controller on your cluster using the following command:
+
+```bash
+kubectl apply -f https://raw.githubusercontent.com/googleforgames/agones/{{< release-branch >}}/examples/custom-controller/deployment.yaml
+```
+
+When you run this command, it quickly sets up your controller by doing four things: 
+ - Sets up the appropriate [RBAC](https://kubernetes.io/docs/reference/access-authn-authz/rbac/) permissions for the custom controller
+ - Launching two controllers for reliability, with leader election setup between them.
+
+## Verify the Controller
+
+To ensure the custom controller is operational, execute the following command. You should see two instances of the controller actively running with the prefix `custom-controller`:
+
+```bash
+kubectl get pods -n agones-system
+```
+
+You should see a successful output similar to this:
+
+```
+NAME                                 READY   STATUS    RESTARTS   AGE
+custom-controller-74c798cfd8-ld6wk   1/1     Running   0          84s
+custom-controller-74c798cfd8-whpp2   1/1     Running   0          84s
+```
+
+## Create a Fleet
+
+Let's create a Fleet using the following command:
+
+```bash
+kubectl apply -f https://raw.githubusercontent.com/googleforgames/agones/{{< release-branch >}}/examples/simple-game-server/fleet.yaml
+```
+
+You should see a successful output similar to this :
+
+```
+fleet.agones.dev/simple-game-server created
+```
+
+This has created a Fleet record inside Kubernetes, which in turn creates two ready [GameServers]({{< ref "/docs/Reference/gameserver.md" >}})
+that are available to be allocated for a game session.
+
+```bash
+kubectl get fleet
+```
+It should look something like this:
+
+```
+NAME                 SCHEDULING   DESIRED   CURRENT   ALLOCATED   READY     AGE
+simple-game-server   Packed       2         3         0           2         9m
+```
+
+You can also see the GameServers that have been created by the Fleet by running `kubectl get gameservers`,
+the GameServer will be prefixed by `simple-game-server`.
+
+```
+NAME                             STATE     ADDRESS            PORT   NODE      AGE
+simple-game-server-llg4x-rx6rc   Ready     192.168.122.205    7752   minikube   9m
+simple-game-server-llg4x-v6g2r   Ready     192.168.122.205    7623   minikube   9m
+```
+
+For the full details of the YAML file head to the [Fleet Specification Guide]({{< ref "/docs/Reference/fleet.md" >}})
+
+{{< alert title="Note" color="info">}} The game servers deployed from a `Fleet` resource will be deployed in the same namespace. The above example omits specifying a namespace, which implies both the `Fleet` and the associated `GameServer` resources will be deployed to the `default` namespace. {{< /alert >}}
+
+## Monitor the log events for the custom controller pod
+
+To monitor the logs of the custom controller during the creation, modification, and deletion of game servers, use the following command:
+
+```bash
+kubectl logs -f deployments/custom-controller -n agones-system
+```
+
+**Note**: If this controller fails for any reason, we've also implemented leader election such that the backup controller will automatically assume the leadership role, ensuring uninterrupted logging of event details.
+
+## Cleaning Up
+
+When you're done with the Agones fleet and the custom controller, it's a good practice to clean up the resources to prevent unnecessary resource consumption. Follow these steps to remove them:
+
+### Remove the Fleet
+
+To delete the Agones fleet you deployed, execute the following command. This will remove the fleet along with all the game server instances it manages:
+
+```bash
+kubectl delete -f https://raw.githubusercontent.com/googleforgames/agones/{{< release-branch >}}/examples/simple-game-server/fleet.yaml
+```
+
+### Remove the Custom Controller
+
+To remove the custom controller from your cluster, execute the following command. This will delete the deployment that you created earlier.
+
+```bash
+kubectl delete -f https://raw.githubusercontent.com/googleforgames/agones/{{< release-branch >}}/examples/custom-controller/deployment.yaml
+```