refactor: migrate i18n files from YAML to TOML format and simplify structure (#1230)

* refactor: migrate i18n files from YAML to TOML format and simplify structure

* refactor: make translation file adjust to gohugoio/locales language code so date translation can work properly

* refactor: use hyphens instead of underscores for i18n, because that's how Hugo pick the translation file

* docs: update comments for i18n language availability and CJK language detection in hugo.toml.

demo/config/_default/languages.toml

@@ -5,18 +5,18 @@
     [en.params.sidebar]
         subtitle = "Example description"
 
-[zh-cn]
+[zh]
     languageName = "简体中文"
     title        = "演示站点"
     weight       = 2
-    [zh-cn.params.sidebar]
+    [zh.params.sidebar]
         subtitle = "演示说明"
 
-[zh-tw]
+[zh-hant-tw]
     languageName = "正體中文"
     title        = "演示站點"
     weight       = 2
-    [zh-tw.params.sidebar]
+    [zh-hant-tw.params.sidebar]
         subtitle = "演示說明"
 
 [ja]