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/tasks/tools/included/kubectl-whats-next.md

@@ -0,0 +1,27 @@
+---
+title: "后续内容"
+description: "安装 kubectl 之后，还可以做些什么？"
+headless: true
+---
+<!-- 
+---
+title: "What's next?"
+description: "What's next after installing kubectl."
+headless: true
+---
+-->
+
+<!-- 
+* [Install Minikube](https://minikube.sigs.k8s.io/docs/start/)
+* See the [getting started guides](/docs/setup/) for more about creating clusters.
+* [Learn how to launch and expose your application.](/docs/tasks/access-application-cluster/service-access-application-cluster/)
+* If you need access to a cluster you didn't create, see the
+  [Sharing Cluster Access document](/docs/tasks/access-application-cluster/configure-access-multiple-clusters/).
+* Read the [kubectl reference docs](/docs/reference/kubectl/kubectl/)
+-->
+* [安装 Minikube](https://minikube.sigs.k8s.io/docs/start/)
+* 有关创建集群的更多信息，请参阅[入门指南](/zh/docs/setup/).
+* [学习如何启动并对外公开你的应用程序。](/zh/docs/tasks/access-application-cluster/service-access-application-cluster/)
+* 如果你需要访问其他人创建的集群，请参阅
+  [共享集群接入文档](/zh/docs/tasks/access-application-cluster/configure-access-multiple-clusters/).
+* 阅读 [kubectl 参考文档](/zh/docs/reference/kubectl/kubectl/)