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/releases/_index.md

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+---
+linktitle: 发行版本历史
+title: 发行版本
+type: docs
+---
+<!-- 
+linktitle: Release History
+title: Releases
+type: docs
+-->
+
+
+<!-- overview -->
+
+<!-- 
+The Kubernetes project maintains release branches for the most recent three minor releases ({{< skew latestVersion >}}, {{< skew prevMinorVersion >}}, {{< skew oldestMinorVersion >}}).  Kubernetes 1.19 and newer receive [approximately 1 year of patch support](/releases/patch-releases/#support-period). Kubernetes 1.18 and older received approximately 9 months of patch support.
+-->
+Kubernetes 项目维护最近三个次要版本（{{< skew latestVersion >}}、{{< skew prevMinorVersion >}}、{{< skew oldMinorVersion >}}）的发布分支。
+Kubernetes 1.19 和更新版本获得[大约 1 年的补丁支持](/zh/releases/patch-releases/#support-period)。
+Kubernetes 1.18 及更早版本获得了大约 9 个月的补丁支持周期。
+
+<!-- 
+Kubernetes versions are expressed as **x.y.z**,
+where **x** is the major version, **y** is the minor version, and **z** is the patch version, following [Semantic Versioning](https://semver.org/) terminology.
+More information in the [version skew policy](/releases/version-skew-policy/) document.
+-->
+
+Kubernetes 版本表示为 **x.y.z**，
+其中 **x** 是主要版本，**y** 是次要版本，**z** 是补丁版本，遵循[语义版本控制](https://semver.org/)术语。
+
+更多信息在[版本偏差策略](/zh/releases/version-skew-policy/)文档中。
+
+<!-- body -->
+
+<!-- ## Release History -->
+## 发行版本历史
+
+{{< release-data >}}
+
+<!-- ## Upcoming Release -->
+## 未来的发行版本
+
+<!-- 
+Check out the [schedule](https://github.com/kubernetes/sig-release/tree/master/releases/release-{{< skew nextMinorVersion >}}) for the upcoming **{{< skew nextMinorVersion >}}** Kubernetes release!
+-->
+查看[时间表](https://github.com/kubernetes/sig-release/tree/master/releases/release-{{< skew nextMinorVersion >}})，
+Kubernetes **{{< skew nextMinorVersion >}}** 版本即将发行！
+
+<!-- ## Helpful Resources -->
+## 有用的资源