docs(v2): update plugins, presets and themes docs (#1889)

* docs(v2): update plugins, presets and themes docs

* ideal image plugin

* proof reading

* Merge master

website/docs/advanced-themes.md

@@ -1,6 +1,6 @@
 ---
 id: advanced-themes
-title: Themes
+title: Writing Themes
 ---
 
 In this doc, we discuss how themes are designed and how you can write your own themes.
@@ -32,47 +32,8 @@ A Docusaurus theme normally includes an `index.js` file where you hook up to the
 ```
 
 There are two lifecycle methods that are essential to theme implementation:
-
-**`getThemePath`**
-
-Returns the path to the directory where the theme components can be found. When your users calls `swizzle`, `getThemePath` is called and its returned path is used to find your theme components.
-
-If you use the folder directory above, your `getThemePath` can be:
-
-```js
-// my-theme/src/index.js
-
-const path = require('path');
-module.exports = function(context, options) {
-  return {
-    name: 'name-of-my-theme',
-    getThemePath() {
-      return path.resolve(__dirname, './theme');
-    },
-  };
-};
-```
-
-**`getClientModules`**
-
-Returns 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.
-
-As an example, to make your theme load a `customCss` object from `options` passed in by the user:
-
-```js
-// my-theme/src/index.js
-
-const path = require('path');
-module.exports = function(context, options) {
-  const {customCss} = options || {};
-  return {
-    name: 'name-of-my-theme',
-    getClientModules() {
-      return [customCss];
-    },
-  };
-};
-```
+- [getThemePath](lifecycle-apis.md#getthemepath)
+- [getClientModules](lifecycle-apis.md#getclientmodules)
 
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