Gen 2 Quickstart guide: add section on deleting app

[TimFinnigan]

Describe the content issue:
A clear and concise description of what the content issue is. Is there content missing or is there incorrect content?

At the end of the Quickstart guide, I think there should be a description (or link to description) on how to delete the app, so that people trying this out do not accumulate unexpected costs.

URL page where content issue is: https://docs.amplify.aws/react/start/quickstart/#-success