Add passkey demo by SK Telecom

Author: Kieun

content/docs/demos-examples/demos.md

@@ -23,3 +23,4 @@ These demo sites have been created and are maintained by FIDO2/WebAuthn vendors
 - [passkeys.io (Hanko)](https://www.passkeys.io/)
 - [passkeys.guru (Descope)](https://passkeys.guru/)
 - [passkey.org (Yubico)](https://passkey.org)
+- [Passkey by SK Telecom (SK Telecom)](https://www.passkey-sktelecom.com/experience)