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/tools/included/optional-kubectl-configs-fish.md

@@ -0,0 +1,31 @@
+---
+title: "fish 自动补全"
+description: "启用 fish 自动补全的可选配置。"
+headless: true
+---
+<!--
+---
+title: "fish auto-completion"
+description: "Optional configuration to enable fish shell auto-completion."
+headless: true
+---
+-->
+
+<!--
+The kubectl completion script for Fish can be generated with the command `kubectl completion fish`. Sourcing the completion script in your shell enables kubectl autocompletion.
+
+To do so in all your shell sessions, add the following line to your `~/.config/fish/config.fish` file:
+-->
+kubectl 通过命令 `kubectl completion fish` 生成 Fish 自动补全脚本。
+在 shell 中导入（Sourcing）该自动补全脚本，将启动 kubectl 自动补全功能。
+
+为了在所有的 shell 会话中实现此功能，请将下面内容加入到文件 `~/.config/fish/config.fish` 中。
+
+```shell
+kubectl completion fish | source
+```
+
+<!--
+After reloading your shell, kubectl autocompletion should be working.
+-->
+重新加载 shell 后，kubectl 自动补全功能将立即生效。