AST Guide: install and migrate tasks needed in new app

Incompatibility of Direct Uploads & Mirror Service

Since #32732 might be fixed only for Rails 6.0, #33255 should be
backported to 5-2-stable directly.

cherry-pick a845ffdb

[Rafael Mendonça França + Stanislas Boyet + bogdanvlviv]

Minor edits to the Active Storage guide

[ci skip] update ActiveStorage documentation

If an explicit AWS key pair and/or region is not provided in
config/storage.yml, attempt to use environment variables, shared
credentials, or IAM role credentials. Order of precedence is
determined by the AWS SDK[1].

[1]: https://docs.aws.amazon.com/sdk-for-ruby/v3/developer-guide/setup-config.html

The Active Storage service for Azure Storage has an option called `path`
that is ambiguous in meaning. It needs to be set to the primary blob
storage endpoint but that can be determined from the blobs client anyway.

To simplify the configuration this commit removes the `path` option and
gets the endpoint from the blobs client instead.

Closes #32225.

Fixes #32129.

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