DCGW: TF how to for outbound DNS

## Jobs to be done (optional)

## Definition of done
- TF how to for creating a network https://github.com/Kong/terraform-provider-konnect/blob/main/examples/scenarios/cloud-gateways.tf (remember: isn't supported for GCP)

## Information
https://app.kapa.ai/93b55f65-cffb-4d0b-9da7-c8915a838dce/top-questions/clusters/1d751e26-2daa-4bee-9236-982724a556be

## Due date (optional)

## Size
L

Start with examples/scenarios/cloud-gateways.tf in the terraform-provider-konnect repository and review the linked Kapa information about outbound DNS. Create a Terraform how-to covering network creation, explicitly noting that the approach is not supported for GCP. Done means the documentation is present, accurate, and includes the example and platform limitation.

Written by the indexing model from the issue text.