DCGW: TF public DCGW how to

## Jobs to be done (optional)

## Definition of done
- TF how to for a public DCGW 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 reviewing the linked Terraform example at terraform-provider-konnect/examples/scenarios/cloud-gateways.tf and the Kapa information link. Determine the documentation location and the steps needed for a public DCGW how-to. Done means the public DCGW workflow is documented and aligned with the referenced Terraform example.

Written by the indexing model from the issue text.