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/debug/node-problem-detector.yaml

@@ -0,0 +1,43 @@
+apiVersion: apps/v1
+kind: DaemonSet
+metadata:
+  name: node-problem-detector-v0.1
+  namespace: kube-system
+  labels:
+    k8s-app: node-problem-detector
+    version: v0.1
+    kubernetes.io/cluster-service: "true"
+spec:
+  selector:
+    matchLabels:
+      k8s-app: node-problem-detector  
+      version: v0.1
+      kubernetes.io/cluster-service: "true"
+  template:
+    metadata:
+      labels:
+        k8s-app: node-problem-detector
+        version: v0.1
+        kubernetes.io/cluster-service: "true"
+    spec:
+      hostNetwork: true
+      containers:
+      - name: node-problem-detector
+        image: k8s.gcr.io/node-problem-detector:v0.1
+        securityContext:
+          privileged: true
+        resources:
+          limits:
+            cpu: "200m"
+            memory: "100Mi"
+          requests:
+            cpu: "20m"
+            memory: "20Mi"
+        volumeMounts:
+        - name: log
+          mountPath: /log
+          readOnly: true
+      volumes:
+      - name: log
+        hostPath:
+          path: /var/log/