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/pods/config/example-redis-config.yaml

@@ -0,0 +1,8 @@
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: example-redis-config
+data:
+  redis-config: |
+    maxmemory 2mb
+    maxmemory-policy allkeys-lru

content/zh-cn/examples/pods/config/redis-config

@@ -0,0 +1,2 @@
+maxmemory 2mb
+maxmemory-policy allkeys-lru

content/zh-cn/examples/pods/config/redis-pod.yaml

@@ -0,0 +1,33 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: redis
+spec:
+  containers:
+  - name: redis
+    image: redis:5.0.4
+    command:
+      - redis-server
+      - "/redis-master/redis.conf"
+    env:
+    - name: MASTER
+      value: "true"
+    ports:
+    - containerPort: 6379
+    resources:
+      limits:
+        cpu: "0.1"
+    volumeMounts:
+    - mountPath: /redis-master-data
+      name: data
+    - mountPath: /redis-master
+      name: config
+  volumes:
+    - name: data
+      emptyDir: {}
+    - name: config
+      configMap:
+        name: example-redis-config
+        items:
+        - key: redis-config
+          path: redis.conf