Add button platform to OpenTherm Gateway (#34557)

* Add opentherm_gw button platform

* Add button description to entities section

source/_integrations/opentherm_gw.markdown

@@ -2,6 +2,7 @@
 title: OpenTherm Gateway
 description: Control your OpenTherm Gateway from Home Assistant.
 ha_category:
+  - Button
   - Binary sensor
   - Climate
   - Hub
@@ -14,6 +15,7 @@ ha_codeowners:
   - '@mvn23'
 ha_domain: opentherm_gw
 ha_platforms:
+  - button
   - binary_sensor
   - climate
   - select
@@ -24,12 +26,6 @@ ha_integration_type: integration
 
 The `opentherm_gw` integration is used to control the [OpenTherm Gateway](https://otgw.tclcode.com/) from Home Assistant.
 
-The following device types are currently supported within Home Assistant:
-
-- Binary sensor
-- Climate
-- Sensor
-
 This integration will add three devices to Home Assistant for each configured gateway. The main control of the integration is a single `climate` entity which can be found on the added `OpenTherm Thermostat` device. All added devices have a collection of `sensor` and `binary_sensor` entities, which are disabled by default. To enable them, follow the steps on [enabling entities](/common-tasks/general/#enabling-or-disabling-entities).
 
 {% note %}
@@ -75,6 +71,10 @@ Floor Temperature:
 
 ## Entities
 
+### Button
+
+The restart button on the `OpenTherm Gateway` device can be used to restart the OpenTherm Gateway.
+
 ### Select
 
 Several `select` configuration entities can be found on the `OpenTherm Gateway` device. These can be used to configure the LEDs and GPIO pins of the OpenTherm Gateway. More information about the available modes can be found in the sections [LED modes](#led-modes) and [GPIO modes](#gpio-modes) or in the [commands documentation](https://otgw.tclcode.com/firmware.html#configuration) of the OpenTherm Gateway.