Update push notification docs

[amhinson]

Fixes: #1791

Description of changes:
Updating Push Notification docs in order to unblock customers going through documentation for the first time.

Overview:

• Remove unnecessary npm & linking install instructions: Since this doc isn't meant to be an end-to-end tutorial, we can assume customers will already have a React Native project already created. Also, since autolinking has been available since RN 0.60+, we can simplify the linking instructions to just use npx pod-install.
• Update Amplify setup: Since Amplify Push Notifications relies on Analytics to already be setup, the amplify init instructions are redundant. Also, amplify push is not needed after running amplify add notifications.
• Update JS configuration example: Customers no longer need to call PushNotification.configure() explicitly, so all of the configuration should be inside Amplify.configure().
• Update Android dependencies & remove unnecessary steps.
• Certain iOS steps were removed, as they were unnecessary for the scope of this library (i.e. manage signing, ask on Launch)

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[sammartinez]

LGTM 🌮