Add a guide for tagging for AWS

[jkodroff]

Add a short guide to `clouds/aws/guides that clearly shows how to add dynamic tags at runtime to all AWS resources (e.g. the current Pulumi org, project, and stack, respectively.

The simplest way to do this today is to use the defaultTags capability of the AWS provider: create an explicit provider and assign it to all resources if not yet assigned.

We should also remove the incomplete example from https://www.pulumi.com/docs/iac/concepts/options/transforms/#stack-transforms.