Multiple stacks: document resource sharing

The current version of [Create an App with Multiple Stacks](https://docs.aws.amazon.com/cdk/latest/guide/stack_how_to_create_multiple_stacks.html) in the Developer Guide does not document how to share resources across multiple stacks.

The CDK EC2 documentation has a pretty good example which could be used in the general dev guide: [Sharing VPCs between stacks
](https://docs.aws.amazon.com/cdk/api/latest/docs/aws-ec2-readme.html#sharing-vpcs-between-stacks)

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This is a 📕 documentation issue

Start with the Developer Guide page “Create an App with Multiple Stacks” linked in the issue, then review the EC2 documentation example for sharing VPCs between stacks. Update the general guide to document resource sharing across multiple stacks, with the sharing example serving as the reference for what done looks like.

Written by the indexing model from the issue text.