Make prev links work in guides and APIs.

Summary:
We had rendering support for prev links, but we never had any previous links in our metadata. Only next links. This adds that support to both Guides and APIs.

**For guides**: `previous` is manually inserted into the metadata of the actual markdown file.
**For APIs/Components**: `previous` is established via code within `extractDocs.js`

> This isn't totally perfect. For example, the transition from the last guide to the first API/component has a next link from the guide, but not a previous link from the API since the way you get the previous links are different from guides and APIs. But this gets us really close.
Closes https://github.com/facebook/react-native/pull/8754

Differential Revision: D3557972

Pulled By: hramos

fbshipit-source-id: e270bb51e7a4f59f61dad28ae0928d27d0af3d4a

docs/Animations.md

@@ -5,6 +5,7 @@ layout: docs
 category: Guides
 permalink: docs/animations.html
 next: accessibility
+previous: handling-touches
 ---
 
 Fluid, meaningful animations are essential to the mobile user experience. Like
@@ -282,7 +283,7 @@ class App extends React.Component {
     this.state = { w: 100, h: 100 };
     this._onPress = this._onPress.bind(this);
   }
-  
+
   componentWillMount() {
     // Animate creation
     LayoutAnimation.spring();