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/overview.md

@@ -0,0 +1,28 @@
+---
+id: plugins-overview
+title: 'Docusaurus plugins'
+sidebar_label: Plugins overview
+slug: '/api/plugins'
+---
+
+We provide official Docusaurus plugins.
+
+## Content plugins
+
+These plugins are responsible to load your site's content, and create pages for your theme to render.
+
+- [@docusaurus/plugin-content-docs](./plugin-content-docs.md)
+- [@docusaurus/plugin-content-blog](./plugin-content-blog.md)
+- [@docusaurus/plugin-content-pages](./plugin-content-pages.md)
+
+## Behavior plugins
+
+These plugins will add a useful behavior to your Docusaurus site.
+
+- [@docusaurus/plugin-debug](./plugin-debug.md)
+- [@docusaurus/plugin-sitemap](./plugin-sitemap.md)
+- [@docusaurus/plugin-pwa](./plugin-pwa.md)
+- [@docusaurus/plugin-client-redirects](./plugin-client-redirects.md)
+- [@docusaurus/plugin-ideal-image](./plugin-ideal-image.md)
+- [@docusaurus/plugin-google-analytics](./plugin-google-analytics.md)
+- [@docusaurus/plugin-google-gtag](./plugin-google-gtag.md)