DCGW: TF how to for private DNS

## Jobs to be done (optional)

## Definition of done
- TF how to for private DNS https://github.com/Kong/terraform-provider-konnect/blob/main/examples/scenarios/cloud-gateways.tf

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

## Due date (optional)

## Size
L

Start by reading examples/scenarios/cloud-gateways.tf in the terraform-provider-konnect repository and the linked Kapa information. Document the private DNS setup as a Terraform how-to for DCGW, with the completed guide covering the requested scenario and matching the repository’s existing examples.

Written by the indexing model from the issue text.