Add pod.yaml to user-guide

This is referenced in k/community for an example
on how to run Kubernetes locally.

It was removed earlier since it was thought as not used.

test/fixtures/doc-yaml/user-guide/pod.yaml

@@ -0,0 +1,12 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: nginx
+  labels:
+    app: nginx
+spec:
+  containers:
+  - name: nginx
+    image: nginx
+    ports:
+    - containerPort: 80