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/generated/kubeadm_certs.md

@@ -0,0 +1,73 @@
+<!--
+The file is auto-generated from the Go source code of the component using a generic
+[generator](https://github.com/kubernetes-sigs/reference-docs/). To learn how
+to generate the reference documentation, please read
+[Contributing to the reference documentation](/docs/contribute/generate-ref-docs/).
+To update the reference conent, please follow the 
+[Contributing upstream](/docs/contribute/generate-ref-docs/contribute-upstream/)
+guide. You can file document formatting bugs against the
+[reference-docs](https://github.com/kubernetes-sigs/reference-docs/) project.
+-->
+
+<!--
+Commands related to handling kubernetes certificates
+-->
+处理 Kubernetes 证书的相关命令
+
+<!--
+### Synopsis
+-->
+### 概要
+
+<!--
+Commands related to handling kubernetes certificates
+-->
+处理 Kubernetes 证书相关的命令
+
+<!--
+### Options
+-->
+### 选项
+
+   <table style="width: 100%; table-layout: fixed;">
+<colgroup>
+<col span="1" style="width: 10px;" />
+<col span="1" />
+</colgroup>
+<tbody>
+
+<tr>
+<td colspan="2">-h, --help</td>
+</tr>
+<tr>
+<!-- td></td><td style="line-height: 130%; word-wrap: break-word;"><p>help for certs</p></td -->
+<td></td><td style="line-height: 130%; word-wrap: break-word;"><!-- help for certs--><p>certs 命令的帮助</p></td>
+</tr>
+
+</tbody>
+</table>
+
+<!--
+### Options inherited from parent commands
+-->
+### 继承于父命令的选项
+
+   <table style="width: 100%; table-layout: fixed;">
+<colgroup>
+<col span="1" style="width: 10px;" />
+<col span="1" />
+</colgroup>
+<tbody>
+
+<tr>
+<td colspan="2">--rootfs string</td>
+</tr>
+<tr>
+<!-- td></td><td style="line-height: 130%; word-wrap: break-word;"><p>[EXPERIMENTAL] The path to the 'real' host root filesystem.</p></td -->
+<td></td><td style="line-height: 130%; word-wrap: break-word;"><p>[实验] 到'真实'主机根文件系统的路径。</p></td>
+</tr>
+
+</tbody>
+</table>
+
+