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/notes.md

@@ -0,0 +1,34 @@
+---
+linktitle: 发行版本说明
+title: 说明
+type: docs
+description: >
+  Kubernetes 发行版本说明。
+sitemap:
+  priority: 0.5
+---
+<!--
+linktitle: Release Notes
+title: Notes
+type: docs
+description: >
+  Kubernetes release notes.
+sitemap:
+  priority: 0.5
+-->
+
+<!-- 
+Release notes can be found by reading the [Changelog](https://github.com/kubernetes/kubernetes/tree/master/CHANGELOG) that matches your Kubernetes version. View the changelog for {{< skew currentVersionAddMinor 0 >}} on [GitHub](https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-{{< skew currentVersionAddMinor 0 >}}.md).
+
+Alternately, release notes can be searched and filtered online at: [relnotes.k8s.io](https://relnotes.k8s.io). View filtered release notes for {{< skew currentVersionAddMinor 0 >}} on [relnotes.k8s.io](https://relnotes.k8s.io/?releaseVersions={{< skew currentVersionAddMinor 0 >}}.0).
+-->
+
+可以通过阅读与你的 Kubernetes 版本对应的
+[Changelog](https://github.com/kubernetes/kubernetes/tree/master/CHANGELOG)
+找到发行版本说明。
+在 [GitHub](https://github.com/kubernetes/kubernetes/blob/master/CHANGELOG/CHANGELOG-{{< skew currentVersionAddMinor 0 >}}.md)
+上查看 {{< skew currentVersionAddMinor 0 >}} 的变更日志。
+
+或者，可以在以下位置在线搜索和筛选发行版本说明：[relnotes.k8s.io](https://relnotes.k8s.io)。
+在 [relnotes.k8s.io](https://relnotes.k8s.io/?releaseVersions={{< skew currentVersionAddMinor 0 >}}.0)
+上查看 {{< skew currentVersionAddMinor 0 >}} 的筛选后的版本说明。