docs(plugin-debug): add plugin reference

Author: meteorlxy

docs/reference/plugin/debug.md

@@ -1,3 +1,5 @@
 # debug
 
-> TODO
+> @vuepress/plugin-debug
+
+This plugin will add a debug component to your site. The component will only take effect in development mode.

docs/zh/reference/plugin/debug.md

@@ -1,3 +1,5 @@
 # debug
 
-> TODO
+> @vuepress/plugin-debug
+
+该插件会给你的站点添加一个 Debug 组件。该组件仅会在开发模式下生效。