Serverless managed cache

## Jobs to be done (optional)

## Definition of done
- How to for managed cache with serverless
- Mention on https://developer.konghq.com/serverless-gateways/
- What on the existing [DCGW managed cache page](https://developer.konghq.com/dedicated-cloud-gateways/managed-cache/) will apply to Serverless?

## Information
DRP: Josh W.
Aha: https://konghq.aha.io/epics/CGW-E-35
PRD: https://docs.google.com/document/d/1lK_v2Sn3BgI6oj5j-NDT3ShA5XviksY3uLXgOgXw5Yc/edit?tab=t.d1zgvgzgq6zi#heading=h.g8t02q85ro0p there has been a new Cloud AI Gateway PRD that is still a draft and WIP. Serverless is being prioritized and PII service could/would be a multi-tenant service that is shared; so this is still TBD

## Due date (optional)

## Size

Start by reading the existing DCGW managed cache page and the serverless gateways page linked in the issue, then review the Aha epic and draft PRD to resolve what applies to Serverless. Done means adding the managed-cache how-to, documenting the applicable DCGW behavior, and mentioning it on the serverless gateways page.

Written by the indexing model from the issue text.