Add section on IdentityModel's Base64Url utility

Added documentation about the Base64Url class in IdentityModel. This section explains its use for encoding and decoding base64 URLs, complementing existing examples.

Author: khalidabuhakmeh

.idea/docs.duendesoftware.com.iml

@@ -37,6 +37,9 @@ bytes = WebEncoders.Base64UrlDecode(b64url);
 
 var text = Encoding.UTF8.GetString(bytes); 
 Console.WriteLine(text);
+```
+
+## IdentityModel's Base64Url
 
 IdentityModel includes the *Base64Url* class to help with
 encoding/decoding: