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/docs/concepts/cluster-administration/certificates.md

@@ -0,0 +1,18 @@
+---
+title: 证书
+content_type: concept
+weight: 20
+---
+<!--
+title: Certificates
+content_type: concept
+weight: 20
+-->
+
+<!-- overview -->
+
+<!--
+To learn how to generate certificates for your cluster, see [Certificates](/docs/tasks/administer-cluster/certificates/).
+-->
+要了解如何为集群生成证书，参阅[证书](/zh/docs/tasks/administer-cluster/certificates/)。
+