Summary:
This PR adds the interactive [React Native Web Player](http://dabbott.github.io/react-native-web-player/) to the docs. The web player is an embeddable iframe which runs React Native code using components from [react-native-web](https://github.com/necolas/react-native-web). For now, it's primarily for educational purposes, since only the basic components are implemented.
Some details:
- The iframe is loaded from MaxCDN using rawgit, locked down to a git tag.
- Asset paths (i.e. images) are resolved relative to `//facebook.github.io/react-native/`
- When viewed on mobile, it falls back to the syntax-highlighted code blocks.
The WebPlayer can be inserted into markdown by using the fences:
```
```ReactNativeWebPlayer
import ...
AppRegistry.registerComponent ...
`` `
```
I didn't actually add the WebPlayer to any docs pages in this PR. That we c
Closes https://github.com/facebook/react-native/pull/8328
Differential Revision: D3471527
Pulled By: lacker
fbshipit-source-id: 704da41cd77e08c7e2bc820557a74d36e88e8eb7
Devin Abbott
Christopher Chedeau
Janic Duplessis
