docs(v2): proofread docs for alpha.20 (#1658)

* docs(v2): proofread docs for alpha.20

* docs(v2): update creating pages doc

* docs(v2): further updates to creating pages doc

* chore(v2): update CHANGELOG

website/docs/plugins-api.md

@@ -0,0 +1,123 @@
+---
+id: plugins-api
+title: Plugins
+---
+
+Plugins are one of the best ways to add functionality to our Docusaurus. Plugins allow third-party developers to extend or modify the default functionality that Docusaurus provides.
+
+## Installing a plugin
+
+A plugin is an npm package, so you install them like other npm packages using npm.
+
+```bash
+yarn add docusaurus-plugin-name
+```
+
+Then you add it in your site's `docusaurus.config.js`'s `plugins` option:
+
+```jsx
+// docusaurus.config.js
+module.exports = {
+  plugins: [
+    '@docusaurus/plugin-content-pages',
+    [
+      // Plugin with options
+      '@docusaurus/plugin-content-blog',
+      {
+        include: ['*.md', '*.mdx'],
+        path: 'blog',
+      },
+    ],
+  ],
+};
+```
+
+Docusaurus can also load plugins from your local directory, you can do something like the following:
+
+```jsx
+// docusaurus.config.js
+const path = require('path');
+
+module.exports = {
+  plugins: [path.resolve(__dirname, '/path/to/docusaurus-local-plugin')],
+};
+```
+
+## Basic Plugin Definition
+
+Plugins are modules which export a function that takes in the context, options and returns a plain JavaScript object that has some properties defined.
+
+For examples, please refer to several [official plugins](https://github.com/facebook/docusaurus/tree/master/packages) created.
+
+```jsx
+const DEFAULT_OPTIONS = {
+  // Some defaults.
+};
+
+// A JavaScript function that returns an object.
+// `context` is provided by Docusaurus. Example: siteConfig can be accessed from context.
+// `opts` is the user-defined options.
+module.exports = function(context, opts) {
+  // Merge defaults with user-defined options.
+  const options = {...DEFAULT_OPTIONS, ...options};
+
+  return {
+    // A compulsory field used as the namespace for directories to cache
+    // the intermediate data for each plugin.
+    // If you're writing your own local plugin, you will want it to
+    // be unique in order not to potentially conflict with imported plugins.
+    // A good way will be to add your own project name within.
+    name: 'docusaurus-my-project-cool-plugin',
+
+    async loadContent() {
+      // The loadContent hook is executed after siteConfig and env has been loaded
+      // You can return a JavaScript object that will be passed to contentLoaded hook
+    },
+
+    async contentLoaded({content, actions}) {
+      // contentLoaded hook is done after loadContent hook is done
+      // actions are set of functional API provided by Docusaurus. e.g: addRoute
+    },
+
+    async postBuild(props) {
+      // after docusaurus <build> finish
+    },
+
+    // TODO
+    async postStart(props) {
+      // docusaurus <start> finish
+    },
+
+    // TODO
+    afterDevServer(app, server) {
+      // https://webpack.js.org/configuration/dev-server/#devserverbefore
+    },
+
+    // TODO
+    beforeDevServer(app, server) {
+      // https://webpack.js.org/configuration/dev-server/#devserverafter
+    },
+
+    configureWebpack(config, isServer) {
+      // Modify internal webpack config. If returned value is an Object, it
+      // will be merged into the final config using webpack-merge;
+      // If the returned value is a function, it will receive the config as the 1st argument and an isServer flag as the 2nd argument.
+    },
+
+    getPathsToWatch() {
+      // Path to watch
+    },
+
+    getThemePath() {
+      // Returns the path to the directory where the theme components can
+      // be found.
+    },
+
+    getClientModules() {
+      // Return an array of paths to the modules that are to be imported
+      // in the client bundle. These modules are imported globally before
+      // React even renders the initial UI.
+    },
+  };
+};
+```