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/contribute/style/content-guide.md

@@ -0,0 +1,141 @@
+---
+title: 文档内容指南
+linktitle: 内容指南
+content_type: concept
+weight: 10
+---
+<!--
+title: Documentation Content Guide
+linktitle: Content guide
+content_type: concept
+weight: 10
+-->
+
+<!-- overview -->
+<!--
+This page contains guidelines for Kubernetes documentation.
+
+If you have questions about what's allowed, join the #sig-docs channel in
+[Kubernetes Slack](https://slack.k8s.io/) and ask!
+
+You can register for Kubernetes Slack at https://slack.k8s.io/.
+
+For information on creating new content for the Kubernetes
+docs, follow the [style guide](/docs/contribute/style/style-guide).
+-->
+本页包含 Kubernetes 文档的一些指南。
+
+如果你不清楚哪些事情是可以做的，请加入到
+[Kubernetes Slack](https://slack.k8s.io/) 的 `#sig-docs` 频道提问！
+你可以在 https://slack.k8s.io 注册到 Kubernetes Slack。
+
+关于为 Kubernetes 文档创建新内容的更多信息，可参考
+[样式指南](/zh/docs/contribute/style/style-guide)。
+
+<!-- body -->
+
+<!--
+## Overview
+
+Source for the Kubernetes website, including the docs, resides in the
+[kubernetes/website](https://github.com/kubernetes/website) repository.
+
+Located in the `kubernetes/website/content/<language_code>/docs` folder, the
+majority of Kubernetes documentation is specific to the [Kubernetes
+project](https://github.com/kubernetes/kubernetes).
+
+## What's allowed
+
+Kubernetes docs allow content for third-party projects only when:
+
+- Content documents software in the Kubernetes project
+- Content documents software that's out of project but necessary for Kubernetes to function
+- Content is canonical on kubernetes.io, or links to canonical content elsewhere
+-->
+## 概述  {#overview}
+
+Kubernetes 网站（包括其文档）源代码位于
+[kubernetes/website](https://github.com/kubernetes/website) 仓库中。
+
+在 `kubernetes/website/content/<语言代码>/docs` 目录下, 绝大多数 Kubernetes
+文档都是特定于 [Kubernetes 项目](https://github.com/kubernetes/kubernetes)的。
+
+## 可以发布的内容  {#what-s-allowed}
+
+只有当以下条件满足时，Kubernetes 文档才允许第三方项目的内容：
+
+- 内容所描述的软件在 Kubernetes 项目内
+- 内容所描述的软件不在 Kubernetes 项目内，却是让 Kubernetes 正常工作所必需的
+- 内容是被 kubernetes.io 域名收编的，或者是其他位置的标准典型内容
+
+<!--
+### Third party content
+
+Kubernetes documentation includes applied examples of projects in the Kubernetes project&mdash;projects that live in the [kubernetes](https://github.com/kubernetes) and
+[kubernetes-sigs](https://github.com/kubernetes-sigs) GitHub organizations.
+
+Links to active content in the Kubernetes project are always allowed.
+
+Kubernetes requires some third party content to function. Examples include container runtimes (containerd, CRI-O, Docker),
+[networking policy](/docs/concepts/extend-kubernetes/compute-storage-net/network-plugins/) (CNI plugins), [Ingress controllers](/docs/concepts/services-networking/ingress-controllers/), and [logging](/docs/concepts/cluster-administration/logging/).
+
+Docs can link to third-party open source software (OSS) outside the Kubernetes project only if it's necessary for Kubernetes to function.
+-->
+### 第三方内容 {#third-party-content}
+
+Kubernetes 文档包含 Kubernetes 项目下的多个项目的应用示例。
+这里的 Kubernetes 项目指的是 [kubernetes](https://github.com/kubernetes) 和
+[kubernetes-sigs](https://github.com/kubernetes-sigs) GitHub 组织
+下的项目。
+
+链接到 Kubernetes 项目中活跃的内容是一直允许的。
+
+Kubernetes 需要某些第三方内容才能正常工作。例如
+容器运行时（containerd、CRI-O、Docker），
+[联网策略](/zh/docs/concepts/extend-kubernetes/compute-storage-net/network-plugins/)
+（CNI 插件），[Ingress 控制器](/zh/docs/concepts/services-networking/ingress-controllers/)
+以及[日志](/zh/docs/concepts/cluster-administration/logging/)等。
+
+只有对应的第三方开源软件（OSS）是运行 Kubernetes 所必需的，才可以在文档中包含
+指向这些 Kubernetes 项目之外的软件的链接。
+
+<!--
+### Dual sourced content
+
+Wherever possible, Kubernetes docs link to canonical sources instead of hosting
+dual-sourced content.
+
+Dual-sourced content requires double the effort (or more!) to maintain
+and grows stale more quickly.
+
+{{< note >}}
+
+If you're a maintainer for a Kubernetes project and need help hosting your own docs,
+ask for help in [#sig-docs on Kubernetes Slack](https://kubernetes.slack.com/messages/C1J0BPD2M/).
+{{< /note >}}
+-->
+### 双重来源的内容  {#dual-sourced-content}
+
+只要有可能，Kubernetes 文档应该指向标准典型的信息源而不是直接托管多重来源的内容。
+
+双重来源的内容需要双倍（甚至更多）的投入才能维护，而且通常很快就会变得停滞不前。
+
+{{< note >}}
+如果你是一个 Kubernetes 项目的维护者，需要帮忙托管你的文档，
+请在 Kubernetes 的 [#sig-docs 频道](https://kubernetes.slack.com/messages/C1J0BPD2M/)
+提出请求。
+{{< /note >}}
+
+<!--
+### More information
+
+If you have questions about allowed content, join the [Kubernetes Slack](https://slack.k8s.io/) #sig-docs channel and ask!
+-->
+### 更多信息  {#more-information}
+
+如果你对允许出现的内容有疑问，请加入到 [Kubernetes Slack](https://slack.k8s.io/)
+的 `#sig-docs` 频道提问！
+
+## {{% heading "whatsnext" %}}
+
+* 阅读[样式指南](/zh/docs/contribute/style/style-guide)。