[3.14] AI Gateway: mapping OAuth client IDs to MCP tools with enforced ACLs

## Overview

Create how-to guide for mapping OAuth client IDs to consumer groups to enforce ACLs on MCP tools. Details:

- Extends existing ACL tutorial: https://developer.konghq.com/mcp/enforce-acls-on-aggregated-mcp-servers/
- Similar pattern to OIDC guide: https://developer.konghq.com/how-to/configure-oidc-with-consumers/

NOTE: Do reach out to the AI Gateway team for a complete demo and Kong Gateway configuration for this how-to

## DoD

- Prepared how-to document
- Update AI MCP Proxy plugin overview
- Cross link the how-to to other MCP ACL how-tos

NOTE: It'll be great to have this how-to ready for release. See my hand-over document for MCP servers I use

## Info

Size: ~M
Person of contact: Wangchong/Spacewander.