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/hugo-shortcodes/example1.md

@@ -0,0 +1,18 @@
+---
+title: 例子 #1
+---
+
+<!--
+title: Example #1
+-->
+<!--
+This is an **example** content file inside the **includes** leaf bundle.
+-->
+这是一个内容文件**示例**，位于一个**includes**叶子包中。
+
+<!--
+Included content files can also contain shortcodes.
+-->
+{{< note >}}
+被包含的内容文件也可以包含短代码。
+{{< /note >}}