Adds Documention for how to run an in-place Agones upgrade

site/content/en/docs/Installation/upgrading.md

@@ -51,6 +51,74 @@ The following are steps to implement this:
 If you are upgrading a single cluster, we recommend creating a maintenance window, in which your game goes offline
 for the period of your upgrade, as there will be a short period in which Agones will be non-responsive during the upgrade.
 
+{{% feature publishVersion="1.43.0" %}}
+#### In-Place Agones Upgrades
+
+{{< alert color="warning" title="Warning" >}}
+Work is ongoing for [In-Place Agones Upgrades](https://github.com/googleforgames/agones/issues/3766),
+and the feature is currently in `Dev`. Please continue to use the multi-cluster strategy for
+production critical upgrades. Feedback on the upcoming `Alpha` and `Beta` releases is appreciated.
+{{< /alert >}}
+
+For In-Place Agones Upgrades we highly recommend installing using Helm. Helm has a significant
+advantage over `install.yaml` in that Helm automatically rolls back the upgrade if the agones-system
+pods are not all successfully running within the designated `--timeout`.
+
+Note: By “controller derived configuration” we mean the parts of the Game Server’s final state that
+are not part of the Game Server spec template that are instead passed to the Game Server through the
+controller, such as the FEATURE_GATES or the Agones SDK Image.
+
+##### Upgrade Existing Fleet (recommended)
+
+1. Run `helm upgrade my-release --install --atomic --wait --timeout 10m --namespace=agones-system`
+with all the appropriate arguments, such a `--version`, for your specific upgrade. Keep in mind that
+`helm upgrade` overwrites all `--set agones.` arguments, so these must be set for each upgrade. See
+[Helm Configuration]({{< relref "./Install Agones/helm.md" >}}) for a list of all available
+configurable parameters.
+2. Wait until the `helm upgrade` is complete.
+3. Kick off rolling update of existing Fleet.
+  3.1 Make any change to the Fleet Spec Template in your fleet.yaml and reapply with
+  `kubectl apply -f fleet.yaml`.
+  3.2 If you have no changes to make to the existing Fleet Spec Template in the fleet.yaml,
+  then add a label to the spec.template.metadata.labels. This has the added advantage that Game
+  Servers can be selected for allocation by label. Reapply with `kubectl apply -f fleet.yaml`.
+```bash
+apiVersion: agones.dev/v1
+kind: Fleet
+metadata:
+  name: example-fleet
+spec:
+  replicas: 2
+  template:
+    metadata:
+      # Adding a label will start a Fleet rollout (RollingUpdate or Recreate based on the replacement
+      # strategy type) with the most up to date Agones config. The label can be any `key: value` pair.
+      labels:
+        release: 1.42.0
+```
+  3.3 This creates a new Game Server Set. Ready Game Servers will shut down and be recreated on the
+  new configuraiton on the new Game Server Set. Allocated Game Servers will use the existing
+  configuration, and once they are set back to Ready, these Game Servers will shut down and be
+  replaced by Ready Game Servers at the new configuration on the new Game Server Set.
+  3.4 The rollout is complete once there is one Game Server Set at the new configuration.
+4. Run any other tests to ensure the Agones installation is working as expected.
+5. Congratulations - you have now upgraded to a new version of Agones! 👍
+
+##### Delay Upgrade to Existing Fleet (not recommended)
+
+1. Run `helm upgrade my-release --install --atomic --wait --timeout 10m --namespace=agones-system`
+with all the appropriate arguments, such a `--version`, for your specific upgrade. Keep in mind that
+`helm upgrade` overwrites all `--set agones.` arguments, so these must be set for each upgrade. See
+[Helm Configuration]({{< relref "./Install Agones/helm.md" >}}) for a list of all available
+configurable parameters.
+2. Continue using Fleet at its existing configuration without kicking off a Fleet upgrade.
+  2.1 Allocated Game servers that return to the Ready state retain the old configuration
+  2.2 Any new Game Servers (either through manually increasing the number of replicas in
+  the fleet.yaml file or through an autoscaler) will be at the new configuration
+3. Run any other tests to ensure the Agones installation is working as expected.
+4. Congratulations - you have now upgraded to a new version of Agones! 👍
+{{% /feature %}}
+
 #### Installation with install.yaml
 
 If you installed [Agones with install.yaml]({{< relref "./Install Agones/yaml.md" >}}), then you will need to delete