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/tasks/tools/included/kubectl-convert-overview.md

@@ -0,0 +1,24 @@
+---
+title: "kubectl-convert 概述"
+description: >-
+  一个 kubectl 插件，允许你将清单从一个 Kubernetes API 版本转换到不同的版本。
+headless: true
+---
+<!--
+---
+title: "kubectl-convert overview"
+description: >-
+  A kubectl plugin that allows you to convert manifests from one version
+  of a Kubernetes API to a different version.
+headless: true
+---
+-->
+
+<!--
+A plugin for Kubernetes command-line tool `kubectl`, which allows you to convert manifests between different API
+versions. This can be particularly helpful to migrate manifests to a non-deprecated api version with newer Kubernetes release.
+For more info, visit [migrate to non deprecated apis](/docs/reference/using-api/deprecation-guide/#migrate-to-non-deprecated-apis)
+-->
+一个 Kubernetes 命令行工具 `kubectl` 的插件，允许你将清单在不同 API 版本间转换。
+这对于将清单迁移到新的 Kubernetes 发行版上未被废弃的 API 版本时尤其有帮助。
+更多信息请访问 [迁移到非弃用 API](/zh/docs/reference/using-api/deprecation-guide/#migrate-to-non-deprecated-apis)