[dahuadoor] Add DHIP UDP multicast auto-discovery service (#20508)

* [dahuadoor] Add DHIP UDP multicast auto-discovery service

Signed-off-by: Sven Schad <svnsssd@gmail.com>

Author: svnsssd

bundles/org.openhab.binding.dahuadoor/README.md

@@ -11,8 +11,12 @@ This binding integrates Dahua VTO Villastation door controllers with openHAB, en
 
 ## Discovery
 
-Automatic discovery is not supported.
-Things must be manually configured.
+Dahua door stations are automatically discovered on the local network using the DHIP UDP multicast discovery protocol.
+The discovered thing is pre-configured with the device's IP address as `hostname`.
+`username` and `password` must be set manually after accepting the thing in the inbox.
+
+**Note:** Auto-discovery relies on UDP multicast (`239.255.255.251:37810`) and therefore only works when openHAB and the Dahua devices are on the **same subnet**.
+Devices in a different subnet or VLAN will not be found automatically and must be added manually.
 
 ## Thing Configuration
 
@@ -57,6 +61,9 @@ Single-button outdoor station.
 
 #### Thing Configuration
 
+When discovered automatically, the thing ID is based on the device serial number when available (e.g., `abc1234xyz56789`), and otherwise falls back to the device MAC address and then the hostname.
+For manual configuration, any unique ID can be used.
+
 ```java
 Thing dahuadoor:vto2202:frontdoor "Front Door Station" @ "Entrance" [
     hostname="192.168.1.100",