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/_index.md

@@ -0,0 +1,30 @@
+---
+title: 任务
+main_menu: true
+weight: 50
+content_type: concept
+---
+<!--
+title: Tasks
+main_menu: true
+weight: 50
+content_type: concept
+-->
+
+<!-- overview -->
+<!--
+This section of the Kubernetes documentation contains pages that
+show how to do individual tasks. A task page shows how to do a
+single thing, typically by giving a short sequence of steps.
+-->
+
+Kubernetes 文档这一部分包含的一些页面展示如何去完成单个任务。
+每个任务页面是一般通过给出若干步骤展示如何执行完成某事。
+
+<!--
+If you would like to write a task page, see
+[Creating a Documentation Pull Request](/docs/contribute/new-content/open-a-pr/).
+-->
+如果你希望编写一个任务页面，参考
+[创建一个文档拉取请求](/zh/docs/contribute/new-content/open-a-pr/)。
+