Add link to Digital Credentials API testing

Author: mohamedamir

index.bs

@@ -15002,3 +15002,5 @@ The following external specifications define additional WebDriver BiDi modules:
 1. <a href="https://wicg.github.io/ua-client-hints/#automation">User-Agent Client Hints</a>
 
 1. <a href="https://webbluetoothcg.github.io/web-bluetooth/#automated-testing">Web Bluetooth</a>
+
+1. <a href="https://www.w3.org/TR/digital-credentials/#automated-testing">Digital Credentials API</a>