Add Algolia search (facebookarchive#1864)

Summary:
**Summary**

Add Algolia search as per facebookarchive#1775.

**Test Plan**

<img width="1552" alt="screen shot 2018-09-12 at 10 07 59 am" src="https://user-images.githubusercontent.com/1315101/45397732-fa33e780-b673-11e8-8264-5044e82fbe61.png">

Clicking on the results brings me to the correct location in the docs. It's magic!
Pull Request resolved: facebookarchive#1864

Reviewed By: niveditc

Differential Revision: D9784511

Pulled By: niveditc

fbshipit-source-id: 4e1b0b35af85ab349d79a83315f2010bc32c7f7f

Author: yangshun

website/siteConfig.js

@@ -47,6 +47,12 @@ const siteConfig = {
 
   // No .html in the path.
   cleanUrl: true,
+
+  // Algolia search.
+  algolia: {
+    apiKey: 'ae94c9e3ee00ea8edddd484adafc37cd',
+    indexName: 'draft-js',
+  },
 };
 
 module.exports = siteConfig;