Replace Navigator references to with existing components

Summary:
Navigator page("docs/navigator.html") does not exist anymore b/c the
Navigator component itself has been removed from the package.

Replace Navigator references to other components that are less likely
to be changed over time(e.g. TextInput and Button components).

No plans to test as this change is on README of the repository.

No issue was created b/c this is not a bug on code nor a feature
request. And there already is an awareness of dead Navigation links
floating inside this repository(e.g. #13804).

Thanks for submitting a PR! Please read these instructions carefully:

- [x] Explain the **motivation** for making this change.
- [x] Provide a **test plan** demonstrating that the code is solid.
- [x] Match the **code formatting** of the rest of the codebase.
- [x] Target the `master` branch, NOT a "stable" branch.

[1]: https://medium.com/martinkonicek/what-is-a-test-plan-8bfc840ec171#.y9lcuqqi9
[2]: https://code.facebook.com/cla
[3]: https://travis-ci.org/facebook/react-native
[4]: http://circleci.com/gh/facebook/react-native
[5]: https://github.com/facebook/react-native/blob/master/CONTRIBUTING.md#pull-requests
Closes #13848

Differential Revision: D5148506

Pulled By: shergin

fbshipit-source-id: e4ecabf05e58efdbe3aca2c4b86e7d884ff6ed62

README.md

@@ -40,11 +40,11 @@ Please use these community resources for getting help. We use the GitHub issues
 
 [The website’s documentation](https://facebook.github.io/react-native/docs/) is divided into multiple sections.
 - There are **Guides** that discuss topics like [debugging](https://facebook.github.io/react-native/docs/debugging.html), [integrating with existing apps](https://facebook.github.io/react-native/docs/integration-with-existing-apps.html), and [the gesture responder system](https://facebook.github.io/react-native/docs/gesture-responder-system.html).
-- The **Components** section covers React components such as [`View`](https://facebook.github.io/react-native/docs/view.html) and [`Navigator`](https://facebook.github.io/react-native/docs/navigator.html).
+- The **Components** section covers React components such as [`View`](https://facebook.github.io/react-native/docs/view.html) and [`Button`](https://facebook.github.io/react-native/docs/button.html).
 - The **APIs** section covers other libraries like [Animated](https://facebook.github.io/react-native/docs/animated.html) and [StyleSheet](https://facebook.github.io/react-native/docs/stylesheet.html) that aren’t React components themselves.
 - Finally, React Native provides a small number of **Polyfills** that offer web-like APIs.
 
-Another great way to learn more about the components and APIs included with React Native is to read their source. Look under the `Libraries` directory for components like `ScrollView` and `Navigator`, for example. The RNTester example is also here to demonstrate some of the ways to use these components. From the source you can get an accurate understanding of each component’s behavior and API.
+Another great way to learn more about the components and APIs included with React Native is to read their source. Look under the `Libraries/Components` directory for components like `ScrollView` and `TextInput`, for example. The RNTester example is also here to demonstrate some of the ways to use these components. From the source you can get an accurate understanding of each component’s behavior and API.
 
 The React Native documentation only discusses the components, APIs and topics specific to React Native (React on iOS and Android). For further documentation on the React API that is shared between React Native and React DOM, refer to the [React documentation](https://facebook.github.io/react/).