Documentation for AWS region selection and credential specification needs improvement

### Describe the issue

The documentation for specifying an AWS region and selecting credentials from shared credential files desperately needs updating in the Rust Developer Guide.

Only one method of specifying credentials is shown on the "_Use a service client in the AWS SDK for Rust_" page.

Anything more advanced than simply loading values from well-known environment variables is not covered.

For example:

* Specify a different profile name in shared credentials file
* Specify a custom file path to shared credentials file
* SAML / SSO workflow

Also, Google searches are riddled with broken code from preview SDK versions, so finding a solution is even harder.

### Links

https://docs.aws.amazon.com/sdk-for-rust/latest/dg/credentials.html
https://docs.aws.amazon.com/sdk-for-rust/latest/dg/client.html