Add Forward Authentication documentation - #415

Adds a new Forward Auth documentation page covering the beta feature that puts an OAuth sign-on in front of applications. The page explains how it works via an in-cluster oauth2-proxy, lists all supported identity providers, and walks through enabling it at both the cluster and service levels — including declarative configuration via porter.yaml. It also documents the X-Auth-Request-User and X-Auth-Request-Email headers forwarded to applications, with a short Express code example. The page is registered in mint.json under the security and compliance section.