docs: Add the documentation for flatcar-sysext

Signed-off-by: Sayan Chowdhury <schowdhury@microsoft.com>

docs/book/src/topics/flatcar.md

@@ -2,9 +2,36 @@
 
 ## Overview
 
-CAPZ enables you to create Kubernetes clusters using Flatcar Container Linux on Microsoft Azure.
+CAPZ enables you to create Kubernetes clusters using Flatcar Container Linux on Microsoft Azure. Flatcar Container Linux comes in two flavors:
 
-### Image creation
+### The `flatcar-sysext` flavor (**recommended**)
+
+This variant relies on a vanilla Flatcar Community Gallery image and it leverages [systemd-sysext](https://www.flatcar.org/docs/latest/provisioning/sysext/) feature to install and update Kubernetes components: the Kubernetes version is not bound to the Flatcar version (i.e Flatcar can be independently upgraded from Kubernetes and vice versa).
+
+The template comes with a [systemd-sysupdate](https://www.freedesktop.org/software/systemd/man/latest/sysupdate.d.html) configuration file that will download each new patch version of Kubernetes (i.e if you start with Kubernetes 1.x.y, systemd-sysupdate will automatically pull 1.x.y+1 but not 1.x+1.y), please note that this behavior is disabled by default. To enable the Kubernetes auto-update you can:
+  * Update the template to enable the `systemd-sysupdate.timer`
+  * Or run the following command on the nodes: `sudo systemctl enable --now systemd-sysupdate.timer`
+
+When the Kubernetes release reaches end-of-life it will not receive updates anymore. To switch to a new major version, do a `sudo rm /etc/sysupdate.kubernetes.d/kubernetes-*.conf` and download the new update config into the folder with `cd /etc/sysupdate.kubernetes.d && sudo wget https://github.com/flatcar/sysext-bakery/releases/download/latest/kubernetes-${KUBERNETES_VERSION%.*}.conf`.
+
+To coordinate the node reboot, we recommend to use [Kured](https://github.com/kubereboot/kured). Note that running `kubeadm upgrade apply` on the first controller and `kubeadm upgrade node` on all other nodes is not automated (yet), see the [docs](https://kubernetes.io/docs/tasks/administer-cluster/kubeadm/kubeadm-upgrade/).
+
+Find the latest published images:
+```console
+az sig image-version list --gallery-image-definition flatcar-stable-amd64 --gallery-name flatcar --resource-group flatcar-image-gallery-publishing -o table
+Location    Name      ProvisioningState    ResourceGroup
+----------  --------  -------------------  --------------------------------
+westeurope  3374.2.0  Succeeded            flatcar-image-gallery-publishing
+westeurope  3374.2.1  Succeeded            flatcar-image-gallery-publishing
+westeurope  3374.2.3  Succeeded            flatcar-image-gallery-publishing
+....
+```
+
+### The `flatcar` flavor
+
+This variant relies on a Flatcar image built using the image-builder project: the Kubernetes version is bound to the Flatcar version and a rebuild of the image is required for each Kubernetes or Flatcar upgrade.
+
+#### Image creation
 
 The testing reference images are built using [image-builder](https://github.com/kubernetes-sigs/image-builder) by Flatcar maintainers and published to the Flatcar CAPI Community Gallery on Azure with community gallery name `flatcar4capi-742ef0cb-dcaa-4ecb-9cb0-bfd2e43dccc0`.
 
@@ -21,7 +48,7 @@ The reference images should not be used in a production environment. It is highl
 Find the latest published images:
 
 ```console
-$ az sig image-definition list-community --location westeurope --public-gallery-name flatcar4capi-742ef0cb-dcaa-4ecb-9cb0-bfd2e43dccc0 --only-show-errors
+$ az sig image-definition list-community --location westeurope --public-gallery-name flatcar4capi-742ef0cb-dcaa-4ecb-9cb0-bfd2e43dccc0 --only-show-errors -o table
 HyperVGeneration    Location    Name                                OsState      OsType    UniqueId
 ------------------  ----------  ----------------------------------  -----------  --------  ---------------------------------------------------------------------------------------------------------------
 V2                  westeurope  flatcar-stable-amd64-capi-v1.23.13  Generalized  Linux     /CommunityGalleries/flatcar4capi-742ef0cb-dcaa-4ecb-9cb0-bfd2e43dccc0/Images/flatcar-stable-amd64-capi-v1.23.13
@@ -36,9 +63,13 @@ False                westeurope  3227.2.3  2022-12-09T18:05:58.830464+00:00  /Co
 
 If you would like customize your images please refer to the documentation on building your own [custom images](custom-images.md).
 
+
 ## Trying it out
 
-To create a cluster using Flatcar Container Linux, use `flatcar` cluster flavor.
+To create a cluster using Flatcar Container Linux, use `flatcar`/`flatcar-sysext` cluster flavor.
 
 - Note: When working with **Flatcar machines**, append `--set-string cloudControllerManager.caCertDir=/usr/share/ca-certificates` to the `cloud-provider-azure` _helm_ command. Refer ["External Cloud Provider's Note for flatcar-flavored machine"](https://github.com/kubernetes-sigs/cluster-api-provider-azure/blob/764aa1e8bd02d150dff90ff6bc7f8daa2b38810f/docs/book/src/topics/addons.md#external-cloud-provider)
-  - However, no changes are needed when using tilt to bring up flatcar-flavored workload clusters.
+  - However, no changes are needed when using tilt to bring up Flatcar workload clusters.
+
+
+