Add section for Cognito Hosted UI on React Native

[manueliglesias]

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One clarification on the two use cases

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Let's add clarification on:

1. Still need to have User Pool/Identity pool in aws_exports
2. Need to configure oauth key: Amplify.Auth.configure({oauth});
3. Linking from expo: import { Linking } from 'expo';
4. In the Cognito console, under URLs for App Client Settings they need to use the URLs that Expo/Metro bundler is hosting the app locally with - e.g. exp://192.168.46.139:19000/--/, http://localhost:3000/, https://myapp.com/. Also need to tell customers what to do in prod by linking to RN docs.
5. Per other comment, explain the difference of the default hosted UI with hostedUISignIn prop vs building your own UI with social provider direct access.
6. Document customProviderSignIn and urlOpener

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Let's get this show on the road!