DCGW: TF/API tabs and refresh of DCGW reference page

## Jobs to be done (optional)

## Definition of done
- Add TF, UI, API tabs for the content on the DCGW reference https://developer.konghq.com/dedicated-cloud-gateways/reference
- Refresh content, some is just outdated (ex. the "public only" note on https://developer.konghq.com/dedicated-cloud-gateways/reference/#securing-backend-communication)

## Information

## Due date (optional)

## Size
L

Start with the DCGW reference page at https://developer.konghq.com/dedicated-cloud-gateways/reference and review the securing-backend-communication section. Identify the source content for the reference page, then add TF, UI, and API tabs and refresh outdated material such as the “public only” note; the page should present current, complete content in all three tabs.

Written by the indexing model from the issue text.