docs: Update Terraform guide to show nested child_modules

[patrick-east]

The current Terraform guide works as expected, but the example only shows using a root_module and no child_modules nested further down.

This is likely to be a common thing that people run into as it is considered good practice to make smaller modules and piece together larger plans with them.

We should update the docs to show an example and give some hints around how to collect all of the resources.

For reference:

• https://www.terraform.io/docs/internals/json-format.html#plan-representation -- Defines the JSON scheme that the plans will have and shows a good example.
• https://play.openpolicyagent.org/p/0K5cSyB6vi -- Shows an example of one way to gather all the resources, as well as example JSON structure.

[princespaghetti]

@patrick-east I'll take a pass at this. Are you thinking an additional example in addition to the blast radius one, or modifying the current one to also have a module included?

[patrick-east]

I think another example, maybe doesn't need to be as complex. Just showing how to deal with collecting resources from the child modules.

[princespaghetti]

Sounds good! I'll start working on that