Gen 2 Quickstart guide: add section on deleting app

**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

Start at the React Gen 2 Quickstart page linked in the issue and locate its source in the documentation repository. Add a section or link at the end explaining how to delete the app and avoid unexpected costs; done means the Quickstart clearly points readers to the deletion instructions.

Written by the indexing model from the issue text.