docs(keycloak): align local WebAuthn admin architecture

[jmgilman]

Summary

• align the Keycloak runtime architecture page with the Flatcar/systemd implementation
• document the local admin + WebAuthn/YubiKey path instead of GitHub as a human IdP
• clarify that the GitHub token broker remains only a machine bootstrap secrets path

Validation

• git diff --check
• npm run typecheck
• npm run build