feat(modbus_controller): Add courtesy response configuration for Modbus server

Author: carlessole

components/modbus_controller.rst

@@ -73,7 +73,18 @@ Configuration variables:
 
 - **max_cmd_retries** (*Optional*, integer): How many times a command will be retried if no response is received. It doesn't include the initial transmition. Defaults to 4.
 
+- **server_courtesy_response** (*Optional*): A configuration block used to enable a courtesy response when acting as a Modbus server.  
+  If present in the configuration, the courtesy response feature is enabled.
+
+  - **register_count** (*Optional*, integer): The highest Modbus register address (inclusive) up to which undefined registers are allowed to be read and will be padded with a default value.  
+    Any read request that includes undefined registers within this range will return the value specified by ``register_value`` instead of triggering an exception.  
+    Defaults to ``65535``.
+
+  - **register_value** (*Optional*, integer): The 16-bit value (range: 0–65535) to return for undefined registers within the address range defined by ``register_count``.  
+    Defaults to ``0``.
+
 - **server_registers** (*Optional*): A list of registers that are responded to when acting as a server.
+
   - **address** (**Required**, integer): start address of the first register in a range
   - **value_type** (*Optional*): datatype of the mod_bus register data. The default data type for ModBUS is a 16 bit integer in **big endian** format (network byte order, MSB first)