Update documentation for building modules

[jsteinich]

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Description

When we expanded module support beyond the Terraform registry (and for submodules), the documentation wasn't really updated to account for that. Both https://github.com/hashicorp/terraform-cdk/blob/main/docs/working-with-cdk-for-terraform/using-providers-and-modules.md and https://github.com/hashicorp/terraform-cdk/blob/main/docs/working-with-cdk-for-terraform/cdktf-json.md don't really explain the new functionality.

References

#584
#563

[github-actions]

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