Windows: Adds RunAsUserName related documentation (#15378)

Recently, the RunAsUserName feature has been added, allowing users to
run Windows containers with custom usernames, different from the image
default usernames.

Adds documentation regarding its prerequisites, feature gate, usage,
restrictions, and samples.

content/en/examples/windows/run-as-username-container.yaml

@@ -0,0 +1,17 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: run-as-username-container-demo
+spec:
+  securityContext:
+    windowsOptions:
+      runAsUserName: "ContainerUser"
+  containers:
+  - name: run-as-username-demo
+    image: mcr.microsoft.com/windows/servercore:ltsc2019
+    command: ["ping", "-t", "localhost"]
+    securityContext:
+        windowsOptions:
+            runAsUserName: "ContainerAdministrator"
+  nodeSelector:
+    beta.kubernetes.io/os: windows

content/en/examples/windows/run-as-username-pod.yaml

@@ -0,0 +1,14 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: run-as-username-pod-demo
+spec:
+  securityContext:
+    windowsOptions:
+      runAsUserName: "ContainerUser"
+  containers:
+  - name: run-as-username-demo
+    image: mcr.microsoft.com/windows/servercore:ltsc2019
+    command: ["ping", "-t", "localhost"]
+  nodeSelector:
+    beta.kubernetes.io/os: windows