Documents ActiveStorage file cleanup in Integration Tests which is
similar but slightly different than the existing docs for System Tests.

Because we use `credentials` in an example of configuration files, but
`credentials` can not be used unless it is 1.8.0 or later.

Ref: 9d65ac30

The [template](https://github.com/rails/rails/blob/master/railties/lib/rails/generators/rails/app/templates/config/storage.yml.tt#L10)
that generates the `config/storage.yml` file has the Amazon S3 key specified as `:amazon`, not `:s3`.

The guides should reflect the nomenclature, given that every other service also
has the name of the company as the key (:google, :microsoft).

Like other Basics and Overview guides.

See #31315 for full discussion

- Describes how to setup each of the services in the `services.yml`
- Integrates copy changes