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/event-exporter.yaml

@@ -0,0 +1,47 @@
+apiVersion: v1
+kind: ServiceAccount
+metadata:
+  name: event-exporter-sa
+  namespace: default
+  labels:
+    app: event-exporter
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: ClusterRoleBinding
+metadata:
+  name: event-exporter-rb
+  labels:
+    app: event-exporter
+roleRef:
+  apiGroup: rbac.authorization.k8s.io
+  kind: ClusterRole
+  name: view
+subjects:
+- kind: ServiceAccount
+  name: event-exporter-sa
+  namespace: default
+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: event-exporter-v0.2.3
+  namespace: default
+  labels:
+    app: event-exporter
+spec:
+  selector:
+    matchLabels:
+      app: event-exporter
+  replicas: 1
+  template:
+    metadata:
+      labels:
+        app: event-exporter
+    spec:
+      serviceAccountName: event-exporter-sa
+      containers:
+      - name: event-exporter
+        image: k8s.gcr.io/event-exporter:v0.2.3
+        command:
+        - '/event-exporter'
+      terminationGracePeriodSeconds: 30