Add "Edit on GitHub" link on normal docs pages

Summary:
<img width="938" alt="screen shot 2016-01-20 at 10 38 23 pm" src="https://cloud.githubusercontent.com/assets/197597/12473376/8f0ac970-bfc6-11e5-9e44-0481e9f250f1.png">
Closes https://github.com/facebook/react-native/pull/5454

Reviewed By: svcscm

Differential Revision: D2849235

Pulled By: vjeux

fb-gh-sync-id: 0103f3d5249b1dfc03aa77feb9ae3bb7bbe9c746

website/layout/DocsLayout.js

@@ -23,7 +23,14 @@ var DocsLayout = React.createClass({
           <DocsSidebar metadata={metadata} />
           <div className="inner-content">
             <a id="content" />
-            <h1>{metadata.title}</h1>
+            <h1>
+              {metadata.title}
+              <a
+                className="edit-github"
+                href={'https://github.com/facebook/react-native/blob/master/docs/' + metadata.filename}>
+                Edit on GitHub
+              </a>
+            </h1>
             <Marked>{content}</Marked>
             <div className="docs-prevnext">
               {metadata.previous && <a className="docs-prev" href={metadata.previous + '.html#content'}>&larr; Prev</a>}