Missing documentation for Single Sign-On (SSO) credentials

### Describe the issue

We are an AWS partner and require SSO to access AWS via Okta. I am writing a Rust application against AWS APIs, which needs to authenticate via SSO.

There is no documentation on how to use SSO with the Rust SDK.

Using the crate reference, I was able to hack something together that compiles, but panics at runtime.

The AWS CLI works fine with SSO, so I know the credentials work alright. I need to get the Rust SDK working though.

```rust
use aws_sdk_s3 as s3;

use s3::Client;

#[tokio::main]
async fn main() {
let sso_region = aws_config::meta::region::RegionProviderChain::default_provider().or_else("us-east-1").region().await;
let sso_creds = aws_config::sso::Builder::new();

let prov_config = aws_config::
let sso_prov = sso_creds
.region(sso_region.unwrap())
.role_name("AdministratorAccess")
.start_url("https://zzzzzzzzzz.awsapps.com/start#")
.account_id("zzzzzzzzz")
.build();

let target_region = aws_config::meta::region::RegionProviderChain::default_provider().or_else("us-west-2").region().await.unwrap();
let config_aws = aws_config::ConfigLoader::default().credentials_provider(sso_prov).region(target_region).load().await;

let s3_client = s3::Client::new(&config_aws);

let bucket_list = s3_client.list_buckets().send().await;

for bucket in bucket_list.unwrap().buckets.unwrap() {
println!("{}", bucket.name.unwrap());
}
}
```

The panic shows this error message, indicating that the Rust SDK is looking for a SSO cache file that doesn't even exist on the local filesystem:

> ProviderError(ProviderError { source: IoError { err: Os { code: 2, kind: NotFound, message: "No such file or directory" }, path: "/Users/zzzzzzz/.aws/sso/cache/zzzzzzzzzz.json" } }), connection: Unknown } })', src/main.rs:23:31
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

How do I authenticate to AWS with SSO, using the Rust SDK?

### Links

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

Start with the linked credentials documentation and the example in src/main.rs around line 23. Determine the supported Rust SDK SSO setup and document the required configuration and cache behavior, with a working authentication example as the completion criterion.

Written by the indexing model from the issue text.