docs: Add custom checkout fields and address formatting documentation

docs/hooks/frontend.mdx

@@ -19,3 +19,11 @@ Modify the standard set of checkout fields recognized by the system.
 
 **Parameters:**
 *   `$keys` (array): List of field aliases (e.g., ['billing_first_name', ...]).
+
+### woonoow_format_address
+
+Allows modifying or overriding the displayed address format (e.g., for My Account address cards and checkout address selection cards). Useful for plugins like RajaOngkir where custom fields replace the default state/city fields.
+
+**Parameters:**
+*   `$formatted_address` (string): The initially formatted address string (usually empty by default).
+*   `$address` (array): The raw address data array containing all standard and custom address fields.