Document data access for modules

[DanielMSchmidt]

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Description

Our documentation is currently spare on details how to access data from modules (HCLModules or generated ones), improving this would help the UX greatly

References

• https://github.com/hashicorp/terraform-cdk/issues/956
• https://github.com/hashicorp/terraform-cdk/issues/1340

[xiehan]

Relates to #2072 - might make sense to tackle both at the same time.