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/blog/_index.md

@@ -0,0 +1,20 @@
+---
+title: Kubernetes 博客
+linkTitle: 博客
+menu:
+  main:
+    title: "博客"
+    weight: 40
+    post: >
+       <p>阅读关于 kubernetes 和容器规范的最新信息,以及获取最新的技术。</p>
+---
+
+{{< comment >}}
+
+<!-- For information about contributing to the blog, see
+https://kubernetes.io/docs/contribute/new-content/blogs-case-studies/#write-a-blog-post -->
+
+有关为博客提供内容的信息，请参见
+https://kubernetes.io/zh/docs/contribute/new-content/blogs-case-studies/#write-a-blog-post
+
+{{< /comment >}}