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/service/networking/networkpolicy.yaml

@@ -0,0 +1,35 @@
+apiVersion: networking.k8s.io/v1
+kind: NetworkPolicy
+metadata:
+  name: test-network-policy
+  namespace: default
+spec:
+  podSelector:
+    matchLabels:
+      role: db
+  policyTypes:
+    - Ingress
+    - Egress
+  ingress:
+    - from:
+        - ipBlock:
+            cidr: 172.17.0.0/16
+            except:
+              - 172.17.1.0/24
+        - namespaceSelector:
+            matchLabels:
+              project: myproject
+        - podSelector:
+            matchLabels:
+              role: frontend
+      ports:
+        - protocol: TCP
+          port: 6379
+  egress:
+    - to:
+        - ipBlock:
+            cidr: 10.0.0.0/24
+      ports:
+        - protocol: TCP
+          port: 5978
+