Add "- React Native" to all pages title in doc section

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Changing all pages in doc to have title " - React Native", which will make the results easier to distinguish like suggested in issue #15659.

I'm not shure how to write a test to this but if you open any page in doc you can see the page title has changed when the title is passed to Site component.
Closes https://github.com/facebook/react-native/pull/15714

Differential Revision: D5739971

Pulled By: TheSavior

fbshipit-source-id: ba9b5ea83fe5c502653972262eb993a24941a62e

website/core/Site.js

@@ -26,7 +26,7 @@ class Site extends React.Component {
     var currentYear = new Date().getFullYear();
 
     var title = this.props.title
-      ? this.props.title
+      ? this.props.title + ' - React Native'
       : 'React Native | A framework for building native apps using React';
 
     var metaTags = [