With pkce reaching 1.0, users on the legacy implicit-flow plugin need a documented migration path. Add a new doc under docs/ (proposed: docs/migrations/legacy-oidc-to-pkce.md), linked
from docs/authorization-configuration.md.

Cover:

• Runtime dep swap: oidc-client v1 → oidc-client-ts v3
• Config: response_type: 'id_token token' → 'code' + response_mode
• Assets folder rename /assets/auth-oidc/ → /assets/auth-oidc-pkce/, updated silent_redirect_uri
• Renamed oidc-client-ts settings keys (shim warns, still works)
• Dropped thirdPartyCookiesScriptLocation
• New logoutUrl vs post_logout_redirect_uri distinction
• Same-origin state validation; errorDescription URL-encoding
• Updated copy-plugin snippet; pointer to the e2e demo config as a working reference