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-content-pages.md

@@ -0,0 +1,68 @@
+---
+id: plugin-content-pages
+title: '📦 plugin-content-pages'
+slug: '/api/plugins/@docusaurus/plugin-content-pages'
+---
+
+The default pages plugin for Docusaurus. The classic template ships with this plugin with default configurations. This plugin provides [creating pages](guides/creating-pages.md) functionality.
+
+## Installation
+
+```bash npm2yarn
+npm install --save @docusaurus/plugin-content-pages
+```
+
+:::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.
+
+:::
+
+## Configuration
+
+```js title="docusaurus.config.js"
+module.exports = {
+  plugins: [
+    [
+      '@docusaurus/plugin-content-pages',
+      {
+        /**
+         * Path to data on filesystem
+         * relative to site dir
+         * components in this directory will be automatically converted to pages
+         */
+        path: 'src/pages',
+        /**
+         * URL route for the page section of your site
+         * do not include trailing slash
+         */
+        routeBasePath: '',
+        include: ['**/*.{js,jsx,ts,tsx,md,mdx}'],
+        /**
+         * No Route will be created for matching files
+         */
+        exclude: [
+          '**/_*.{js,jsx,ts,tsx,md,mdx}',
+          '**/*.test.{js,ts}',
+          '**/__tests__/**',
+        ],
+        /**
+         * Theme component used by markdown pages.
+         */
+        mdxPageComponent: '@theme/MDXPage',
+        /**
+         * Remark and Rehype plugins passed to MDX
+         */
+        remarkPlugins: [],
+        rehypePlugins: [],
+        /**
+         * Custom Remark and Rehype plugins passed to MDX before
+         * the default Docusaurus Remark and Rehype plugins.
+         */
+        beforeDefaultRemarkPlugins: [],
+        beforeDefaultRehypePlugins: [],
+      },
+    ],
+  ],
+};
+```