docs(v2): Docs refactoring and reorganization (#3831)

* stable refactor of plugins api documentation

* plugins sidebar not collapsed by default as small?

* theme docs reorg

* Refactor migration guide doc

* fix broken link

website/docs/api/plugins/plugin-debug.md

@@ -0,0 +1,39 @@
+---
+id: plugin-debug
+title: '📦 plugin-debug'
+slug: '/api/plugins/@docusaurus/plugin-debug'
+---
+
+The debug plugin will display useful debug information at [http://localhost:3000/\_\_docusaurus/debug](http://localhost:3000/__docusaurus/debug).
+
+It is mostly useful for plugin authors, that will be able to inspect more easily the content of the `.docusaurus` folder (like the creates routes), but also be able to inspect data structures that are never written to disk, like the plugin data loaded through the `contentLoaded` lifecycle.
+
+:::note
+
+If you report a bug, we will probably ask you to have this plugin turned on in the production, so that we can inspect your deployment config more easily.
+
+If you don't have any sensitive information, you can keep it on in production [like we do](http://v2.docusaurus.io/__docusaurus/debug).
+
+:::
+
+## Installation
+
+```bash npm2yarn
+npm install --save @docusaurus/plugin-debug
+```
+
+:::tip
+
+If you have installed `@docusaurus/preset-classic`, you don't need to install it as a dependency. You can also configure it through the [classic preset options](presets.md#docusauruspreset-classic) instead of doing it like below.
+
+By default, it's enabled in dev, and disabled in prod, to avoid exposing potentially sensitive information.
+
+:::
+
+## Configuration
+
+```js title="docusaurus.config.js"
+module.exports = {
+  plugins: ['@docusaurus/plugin-debug'],
+};
+```