Switch language name 'zh' to 'zh-cn'

This is the first step to rename 'zh' to 'zh-cn'. There are several reasons why we rename the language name.

- The upstream docsy theme changed the language name, leading to many warnings during site build;
  The side-effect is that the i18n strings are no longer working.
- We believe renaming the language is the right thing to do, because this move can make room for other variants of Chinese language, such as 'zh-tw', 'zh-sg' etc.

There would be several follow-ups to this PR, such as fixing the intra-site links, adding redirects etc.

We will lock up changes to zh/zh-cn pages for the moment, until this one gets in.

This PR is based on commit cdad0a7342.

content/zh-cn/examples/windows/configmap-pod.yaml

@@ -0,0 +1,31 @@
+kind: ConfigMap
+apiVersion: v1
+metadata:
+  name: example-config
+data:
+  example.property.1: hello
+  example.property.2: world
+
+---
+
+apiVersion: v1
+kind: Pod
+metadata:
+  name: configmap-pod
+spec:
+  containers:
+  - name: configmap-redis
+    image: redis:3.0-nanoserver
+    env:
+      - name: EXAMPLE_PROPERTY_1
+        valueFrom:
+          configMapKeyRef:
+            name: example-config
+            key: example.property.1
+      - name: EXAMPLE_PROPERTY_2
+        valueFrom:
+          configMapKeyRef:
+            name: example-config
+            key: example.property.2
+  nodeSelector:
+    kubernetes.io/os: windows