Reorganise the sdk installation sidebar

Summary:
Give the sdk installation pages a bit more structure.
To me it's much clearer now what they are.

Happy to hear differing opinions so we get it right :)

Reviewed By: passy

Differential Revision: D25638164

fbshipit-source-id: 86b7fecd8aba0068336b15a308f4a9433817c68e

docs/getting-started/react-native.mdx

@@ -1,7 +1,7 @@
 ---
 id: react-native
 title: Set up your React Native App
-sidebar_label: React Native
+sidebar_label: Automatic Setup
 ---
 import useBaseUrl from '@docusaurus/useBaseUrl';
 import Link from '@docusaurus/Link';