docs: describe the OpenID Provider in the README #568

The seamless-auth-api README has no mention of OpenID or OIDC. The OpenID Provider shipped in v0.21 and is the widest integration path Seamless Auth has (any OIDC app signs in with no Seamless SDK), but a stranger arriving at the main repo cannot see that it exists.

Found in the 2026-11-02 adoption report (fellscode/docs/seamless/adoption/2026-11-02.md, ranked fix 2).

Done when

• The README says, near "Start here", that the API can act as an OpenID Provider through the separate public login edge in seamless-auth-oidc-provider, off by default
• It links the OpenID Provider guide (https://docs.seamlessauth.com/build/openid-provider/) and the provider repo
• It is honest that the provider is new and not OpenID certified yet, and that the OpenID conformance suite runs in CI
• The architecture diagram, or a second one, shows where the provider edge sits for OIDC clients