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-blog.md

@@ -0,0 +1,107 @@
+---
+id: plugin-content-blog
+title: '📦 plugin-content-blog'
+slug: '/api/plugins/@docusaurus/plugin-content-blog'
+---
+
+Provides the [Blog](blog.md) feature and is the default blog plugin for Docusaurus.
+
+## Installation
+
+```bash npm2yarn
+npm install --save @docusaurus/plugin-content-blog
+```
+
+:::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-blog',
+      {
+        /**
+         * Path to data on filesystem relative to site dir.
+         */
+        path: 'blog',
+        /**
+         * URL for editing a blog post.
+         * Example: 'https://github.com/facebook/docusaurus/edit/master/website/blog/'
+         */
+        editUrl:
+          'https://github.com/facebook/docusaurus/edit/master/website/blog/',
+        /**
+         * Blog page title for better SEO
+         */
+        blogTitle: 'Blog title',
+        /**
+         * Blog page meta description for better SEO
+         */
+        blogDescription: 'Blog',
+        /**
+         * Number of blog post elements to show in the blog sidebar
+         * 'ALL' to show all blog posts
+         * 0 to disable
+         */
+        blogSidebarCount: 5,
+        /**
+         * Title of the blog sidebar
+         */
+        blogSidebarTitle: 'All our posts',
+        /**
+         * URL route for the blog section of your site.
+         * *DO NOT* include a trailing slash.
+         */
+        routeBasePath: 'blog',
+        include: ['*.md', '*.mdx'],
+        postsPerPage: 10,
+        /**
+         * Theme components used by the blog pages.
+         */
+        blogListComponent: '@theme/BlogListPage',
+        blogPostComponent: '@theme/BlogPostPage',
+        blogTagsListComponent: '@theme/BlogTagsListPage',
+        blogTagsPostsComponent: '@theme/BlogTagsPostsPage',
+        /**
+         * Remark and Rehype plugins passed to MDX.
+         */
+        remarkPlugins: [
+          /* require('remark-math') */
+        ],
+        rehypePlugins: [],
+        /**
+         * Custom Remark and Rehype plugins passed to MDX before
+         * the default Docusaurus Remark and Rehype plugins.
+         */
+        beforeDefaultRemarkPlugins: [],
+        beforeDefaultRehypePlugins: [],
+        /**
+         * Truncate marker, can be a regex or string.
+         */
+        truncateMarker: /<!--\s*(truncate)\s*-->/,
+        /**
+         * Show estimated reading time for the blog post.
+         */
+        showReadingTime: true,
+        /**
+         * Blog feed.
+         * If feedOptions is undefined, no rss feed will be generated.
+         */
+        feedOptions: {
+          type: '', // required. 'rss' | 'feed' | 'all'
+          title: '', // default to siteConfig.title
+          description: '', // default to  `${siteConfig.title} Blog`
+          copyright: '',
+          language: undefined, // possible values: http://www.w3.org/TR/REC-html40/struct/dirlang.html#langcodes
+        },
+      },
+    ],
+  ],
+};
+```