Fix developer guide URL (#1262)

* Update README.md to fix invalid Developer Guide URL

The old guide:
https://github.com/awslabs/aws-sdk-rust/blob/main/Guide.md

has been deleted and changed to:
https://docs.aws.amazon.com/sdk-for-rust/latest/dg/welcome.html

See: https://github.com/awslabs/aws-sdk-rust/pull/325



Signed-off-by: bin liu <liubin0329@gmail.com>

* Update invalid Developer Guide URL in AwsReadmeDecorator.kt

The old guide:
https://github.com/awslabs/aws-sdk-rust/blob/main/Guide.md

has been deleted and changed to:
https://docs.aws.amazon.com/sdk-for-rust/latest/dg/welcome.html

See: https://github.com/awslabs/aws-sdk-rust/pull/325



Signed-off-by: bin liu <liubin0329@gmail.com>

* Add fixing Developer Guide link to  CHANGELOG.next.toml

Signed-off-by: bin liu <liubin0329@gmail.com>

Co-authored-by: Russell Cohen <rcoh@amazon.com>