Migrate and adapt OIDC policy overview

## Overview

As the OIDC policy has been de-listed and merged into the AI Auth Strategies entity schema, we need to create an **OIDC reference page** as part of the AI Auth Strategies entity documentation.

Reuse and adapt the relevant conceptual content from the existing OIDC documentation. The new page should explain how OIDC authentication works when configured as an Auth Strategy and how it works with AI Gateway 2.0, rather than documenting the standalone policy/plugin.

The existing OIDC documentation covers a much broader feature set, including numerous authentication flows and grants, authorization mechanisms, and advanced functionality, so only the subset supported by Auth Strategies should be migrated.

## Content to migrate and/or adapt (TBD)

* How OIDC authentication with an identity provider works
* Supported authentication/token validation flow(s)
* OIDC discovery and JWKS behavior, where applicable
* Token introspection, where applicable
* Claims validation and mapping
* AI Consumer/identity mapping, where applicable
* Upstream identity/token behavior
* Supported identity providers
* Relevant examples, rewritten for Auth Strategies

## Notes

Only carry over functionality that is actually supported by the Auth Strategy implementation. In particular, validate which of the broader OIDC policy capabilities, such as authorization code, sessions, grants, token exchange, multi-IdP, and proof-of-possession apply to AI Auth Strategies before migrating them.

TBD, but likely alongside or underneath the AI Auth Strategies entity reference.

## DoD

* Useful OIDC reference content is reused rather than recreated from scratch.
* Content is adapted to the Auth Strategies model and terminology.
* Policy/plugin-specific configuration, links, and examples are removed or replaced.
* The page is a conceptual/configuration reference, not an API field reference.
* Only OIDC functionality supported by AI Auth Strategies is documented.
* Differences or limitations compared with the OIDC plugin/policy are clearly represented where relevant.