(aws-cdk-lib): Better Document Stages

### Describe the issue

A common concept in a lot of recent frameworks in the concept of "Stacks" or "Stages" as discreet environment-specific configurations of Stacks.

The CDK even has the concept of [Stages](https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.Stage.html), but it's largely relegated to documenting it's usage in the context of CDK Pipelines.

I'd love to see the documentation expanded to describe how one would use stages in a manner similar to Serverless Framework, SST, or Pulumi Stacks, and how to deploy specific stages via the CLI without using pipelines.

### Links

https://docs.aws.amazon.com/cdk/v2/guide/stacks.html
https://docs.aws.amazon.com/cdk/v2/guide/environments.html

Start with the linked CDK stacks and environments guide pages and review how Stage usage is currently documented for CDK Pipelines. Done means the documentation explains framework-style, environment-specific stages and how to deploy a selected stage through the CLI without pipelines.

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