Add tabs to Getting Started and update Android Studio steps

Summary:
There are two ways to get started with React Native: using CRNA, or by building native code. The latter requires a set of dependencies that also need to be installed even by people who use CRNA if they have ejected.

There needs to be a clear demarcation between the two approaches as well as clear instructions as to which steps are of interest to CRNA using parties or people adding RN to an existing app.

Separating the two approaches entirely by using tabs can achieve this purpose.

Built and tested the website on Chrome on macOS. Have not yet tested other browsers or OS configurations.

![screencapture-localhost-8079-react-native-docs-getting-started-html-1494974542733](https://cloud.githubusercontent.com/assets/165856/26131374/77258b0c-3a4e-11e7-9a17-074610ca33d1.png)

![screencapture-localhost-8079-react-native-docs-getting-started-html-1494974580882](https://cloud.githubusercontent.com/assets/165856/26131370/73806990-3a4e-11e7-9e67-73c519b050da.png)

This is a work in progress. Feedback is appreciated. I am not yet happy with the different sets of tabs/buttons, for example.
Closes facebook#14005

Differential Revision: D5096426

Pulled By: hramos

fbshipit-source-id: cdbf19b148ec87f2f2dfdced345084d0bbff4c26

Author: hramos