DCGW: Updates to AWS private zones

## Definition of done
- https://developer.konghq.com/dedicated-cloud-gateways/private-hosted-zones/ should only be UI instructions (this is how most people are configuring this)
- The VPC ID is hard to find, in [this prereq](https://developer.konghq.com/dedicated-cloud-gateways/private-hosted-zones/#aws-private-hosted-zone) add a link to https://docs.aws.amazon.com/managedservices/latest/userguide/find-vpc.html
- We set the region as an env var in the prereq, but don't use [it here](https://developer.konghq.com/dedicated-cloud-gateways/private-hosted-zones/#associate-konnect-with-your-private-hosted-zone). Convert the region to an env var (and test that it works).
- I didn't have to use [this AWS CLI step](https://developer.konghq.com/dedicated-cloud-gateways/private-hosted-zones/#associate-konnect-with-your-private-hosted-zone) when I configured mine, maybe because I used an existing VPC? Verify that this step is needed with the DCGW team.
- https://developer.konghq.com/dedicated-cloud-gateways/reference/ add a section for a Private DNS reference that contains info about what it is, why you'd use it, and UI, API, and [Terraform](https://github.com/Kong/terraform-provider-konnect/blob/main/examples/resources/cloud_gateway_private_dns.tf) instructions to configure it generically.

## Information
SME: Josh W. and the DCGW team

## Size
Medium

Start with the private hosted zones page and the reference page, then review the linked AWS VPC guidance and Terraform example. Confirm the AWS CLI step with Josh W. and the DCGW team, and verify the region environment variable works. Done means the existing page is UI-focused, prerequisites include the VPC link, and the reference page covers private DNS with UI, API, and Terraform instructions.

Written by the indexing model from the issue text.