Create guide and evals for the passkeys use case

[chrome-devrel-infra]

Comprehensive orientation and cross-cutting principles for implementing WebAuthn and Passkeys in web applications. Use this guide when handling passkey registration, authentication, management, or reauthentication.

Affected web-feature IDs: webauthn

Use case subdir: guides/security/passkeys

Required files:

• Use case metadata (guide.md frontmatter)
• demo.html
• Use case guidance (guide.md)
• expectations.md
• tasks/task.md
• negative-demo.html
• grader.ts