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.

config.toml

@@ -302,15 +302,15 @@ languageName = "English"
 weight = 1
 languagedirection = "ltr"
 
-[languages.zh]
+[languages.zh-cn]
 title = "Kubernetes"
 description = "生产级别的容器编排系统"
 languageName = "中文 Chinese"
 weight = 2
-contentDir = "content/zh"
+contentDir = "content/zh-cn"
 languagedirection = "ltr"
 
-[languages.zh.params]
+[languages.zh-cn.params]
 time_format_blog = "2006.01.02"
 language_alternatives = ["en"]