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/reference/setup-tools/kubeadm/kubeadm-alpha.md

@@ -0,0 +1,42 @@
+---
+title: kubeadm alpha
+content_type: concept
+weight: 90
+---
+<!--
+title: kubeadm alpha
+content_type: concept
+weight: 90
+-->
+
+{{< caution >}}
+<!--
+`kubeadm alpha` provides a preview of a set of features made available for gathering feedback
+ from the community. Please try it out and give us feedback!
+ -->
+`kubeadm alpha` 提供了一组可用于收集社区反馈的预览性质功能。
+请试用这些功能并给我们提供反馈！
+{{< /caution >}}
+
+<!--
+Currently there are no experimental commands under `kubeadm alpha`.
+-->
+目前在 `kubeadm alpha` 之下没有试验性质的命令。
+
+## {{% heading "whatsnext" %}}
+
+<!--
+* [kubeadm init](/docs/reference/setup-tools/kubeadm/kubeadm-init/) to bootstrap a Kubernetes control-plane node
+* [kubeadm join](/docs/reference/setup-tools/kubeadm/kubeadm-join/) to connect a node to the cluster
+* [kubeadm reset](/docs/reference/setup-tools/kubeadm/kubeadm-reset/) to revert any changes made to this host by `kubeadm init` or `kubeadm join`
+-->
+* 用来启动引导 Kubernetes 控制平面节点的
+  [kubeadm init](/zh/docs/reference/setup-tools/kubeadm/kubeadm-init/)
+  命令
+* 用来将节点连接到集群的
+  [kubeadm join](/zh/docs/reference/setup-tools/kubeadm/kubeadm-join/) 
+  命令
+* 用来还原 `kubeadm init` 或 `kubeadm join` 操作对主机所做的任何更改的
+  [kubeadm reset](/zh/docs/reference/setup-tools/kubeadm/kubeadm-reset/)
+  命令
+