Summary:
Finally, a place where `Button` is properly introduced. This is based on the old Handling Touches guide, which has been simplified (with some content moved over to the scroll views tutorial).
I've also updated the ordering of the guides into something that makes more sense to someone just getting started with React Native.
Closes https://github.com/facebook/react-native/pull/14371
Differential Revision: D5201127
Pulled By: hramos
fbshipit-source-id: 819192e2db9febb8a315f51693dae557752b6002
f42f2dff37