Signer Abstraction: Document deny stage configuration for wallet ID mismatch with Organization ID

[mariacretu-arch]

Document deny stage configuration for wallet ID mismatch with Organization ID for Data Space Operator Authentik instance which supports multiple users from different organizations.
Mention that for user registration flow, signer_url, walletID and org ID are needed to be passed for Central Authentik instance. Federated IdP will require same fields for registration excluding denying stage, because wallet IDs will be valid for that specific Participant.