Add Terraform and UI example for Serverless

## Definition of done

- Add a Terraform tab to the [following page](https://developer.konghq.com/serverless-gateways/reference/#how-do-i-provision-a-serverless-gateway) with a setup for serverless (use this example: https://github.com/Kong/terraform-provider-konnect/blob/main/examples/scenarios/serverless-gateway.tf)
- Also add a UI tab with UI instructions to configure serverless on the same page

## Information
Kapa ID: 009609ef-2fd9-4cdc-aed4-b6ca927df952

## Size
Medium

Start with the serverless gateway provisioning section at the referenced developer.konghq.com page and review the linked Terraform example in terraform-provider-konnect. Add Terraform and UI tabs covering serverless setup, then verify both tabs appear on the same page and their instructions satisfy the definition of done.

Written by the indexing model from the issue text.