chore: keep core documentation in sync between @aws-cdk/core and aws-…

…cdk-lib (#14280)

Users of CDKv1 use the @aws-cdk/core documentation as the landing point for
documentation on core concepts like stacks, durations, references, etc. For
CDKv2 users, the same landing point is aws-cdk-lib, which currently has very
limited documentation.

This change introduces a pkglint rule to keep a subset of the two READMEs in
sync. This was chosen over syncing the entire README as the titles, heading,
stability markings, etc. needed to be different between the two packages.


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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/core/README.md

@@ -19,6 +19,8 @@ Guide](https://docs.aws.amazon.com/cdk/latest/guide/home.html) for
 information of most of the capabilities of this library. The rest of this
 README will only cover topics not already covered in the Developer Guide.
 
+<!--BEGIN CORE DOCUMENTATION-->
+
 ## Stacks and Stages
 
 A `Stack` is the smallest physical unit of deployment, and maps directly onto
@@ -911,3 +913,5 @@ When deploying to AWS CloudFormation, it needs to keep in check the amount of re
 It's possible to synthesize the project with more Resources than the allowed (or even reduce the number of Resources).
 
 Set the context key `@aws-cdk/core:stackResourceLimit` with the proper value, being 0 for disable the limit of resources.
+
+<!--END CORE DOCUMENTATION-->