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/setup/learning-environment/_index.md

@@ -0,0 +1,13 @@
+---
+title: 学习环境
+weight: 20
+---
+
+<!--
+{{/* There is a Netlify redirect from this page to /docs/tasks/tools/ */}}
+{{/* This page content only exists to provide a navigation stub */}}
+{{/* and to protect in case that redirect is one day removed. */}}
+
+{{/* If you're localizing this page, you only need to copy the front matter */}}
+{{/* and add a redirect into "/static/_redirects", for YOUR localization. */}}
+-->