Split up getting started docs

Summary:
This is a first step in breaking up the getting started docs.
Ultimately, the React Native Android and iOS bits will become
separate sections, too, but I won't to keep the diffs a bit smaller.
This one doesn't really do much apart from reorganising the current
content.

Reviewed By: mweststrate

Differential Revision: D20815235

fbshipit-source-id: 5c85b788211699a2a9f5808b9e2590c7db68a1c1

website/siteConfig.js

@@ -17,7 +17,7 @@ const siteConfig = {
   projectName: 'flipper',
   headerLinks: [
     {doc: 'features/index', label: 'Features'},
-    {doc: 'getting-started', label: 'Setup'},
+    {doc: 'getting-started/index', label: 'Setup'},
     {doc: 'extending/index', label: 'Extending'},
     {href: repoUrl, label: 'GitHub'},
   ],