Add registration and authentication sequence diagrams to the Passkey Security Cheat Sheet

Suggested by @mackowski while reviewing #2369: add one sequence diagram for the registration ceremony and one for the authentication ceremony to the Passkey Security Cheat Sheet, showing which party (relying party server, client or browser, authenticator) performs each check and in what order. That makes the steps in "Verify the Registration Response" and "Verify the Authentication Response" easier to follow.

Diagrams go in assets/ as PNG and are referenced from the sheet, per CONTRIBUTING.md. Keep them small and readable. Open to anyone who wants to pick it up.