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-kubeconfig.md

@@ -0,0 +1,33 @@
+---
+title: kubeadm kubeconfig
+content_type: concept
+weight: 90
+---
+
+<!--
+`kubeadm kubeconfig` provides utilities for managing kubeconfig files.
+
+For examples on how to use `kubeadm kubeconfig user` see
+[Generating kubeconfig files for additional users](/docs/tasks/administer-cluster/kubeadm/kubeadm-certs#kubeconfig-additional-users).
+-->
+`kubeadm kubeconfig` 提供用来管理 kubeconfig 文件的工具。
+
+如果希望查看如何使用 `kubeadm kubeconfig user` 的示例，请参阅
+[为其他用户生成 kubeconfig 文件](/zh/docs/tasks/administer-cluster/kubeadm/kubeadm-certs#kubeconfig-additional-users).
+
+## kubeadm kubeconfig {#cmd-kubeconfig}
+
+{{< tabs name="tab-kubeconfig" >}}
+{{< tab name="overview" include="generated/kubeadm_kubeconfig.md" />}}
+{{< /tabs >}}
+
+## kubeadm kubeconfig user {#cmd-kubeconfig-user}
+
+<!--
+This command can be used to output a kubeconfig file for an additional user.
+-->
+此命令可用来为其他用户生成一个 kubeconfig 文件。
+
+{{< tabs name="tab-kubeconfig-user" >}}
+{{< tab name="user" include="generated/kubeadm_kubeconfig_user.md" />}}
+{{< /tabs >}}