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/controllers/fluentd-daemonset-update.yaml

@@ -0,0 +1,52 @@
+apiVersion: apps/v1
+kind: DaemonSet
+metadata:
+  name: fluentd-elasticsearch
+  namespace: kube-system
+  labels:
+    k8s-app: fluentd-logging
+spec:
+  selector:
+    matchLabels:
+      name: fluentd-elasticsearch
+  updateStrategy:
+    type: RollingUpdate
+    rollingUpdate:
+      maxUnavailable: 1
+  template:
+    metadata:
+      labels:
+        name: fluentd-elasticsearch
+    spec:
+      tolerations:
+      # 这些容忍度使得守护进程能够在控制平面节点上运行
+      # 如果你的控制平面节点不应该运行 pod，请删除它们
+      - key: node-role.kubernetes.io/control-plane
+        operator: Exists
+        effect: NoSchedule
+      - key: node-role.kubernetes.io/master
+        operator: Exists
+        effect: NoSchedule
+      containers:
+      - name: fluentd-elasticsearch
+        image: quay.io/fluentd_elasticsearch/fluentd:v2.5.2
+        resources:
+          limits:
+            memory: 200Mi
+          requests:
+            cpu: 100m
+            memory: 200Mi
+        volumeMounts:
+        - name: varlog
+          mountPath: /var/log
+        - name: varlibdockercontainers
+          mountPath: /var/lib/docker/containers
+          readOnly: true
+      terminationGracePeriodSeconds: 30
+      volumes:
+      - name: varlog
+        hostPath:
+          path: /var/log
+      - name: varlibdockercontainers
+        hostPath:
+          path: /var/lib/docker/containers