Add MQTT Scenes Docs (#15497)

Co-authored-by: Franck Nijhof <git@frenck.dev>

source/_integrations/scene.mqtt.markdown

@@ -0,0 +1,132 @@
+---
+title: "MQTT Scene"
+description: "Instructions on how to integrate MQTT scenes into Home Assistant."
+ha_category:
+  - Scene
+ha_release: 0.119
+ha_iot_class: Configurable
+ha_domain: mqtt
+---
+
+The `mqtt` scene platform lets you control your MQTT enabled scenes.
+
+## Configuration
+
+To enable a MQTT scene in your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+scene:
+  - platform: mqtt
+    command_topic: zigbee2mqtt/living_room_group/set
+```
+
+{% configuration %}
+availability:
+  description: A list of MQTT topics subscribed to receive availability (online/offline) updates. Must not be used together with `availability_topic`.
+  required: false
+  type: list
+  keys:
+    payload_available:
+      description: The payload that represents the available state.
+      required: false
+      type: string
+      default: online
+    payload_not_available:
+      description: The payload that represents the unavailable state.
+      required: false
+      type: string
+      default: offline
+    topic:
+      description: An MQTT topic subscribed to receive availability (online/offline) updates.
+      required: true
+      type: string
+availability_topic:
+  description: The MQTT topic subscribed to receive availability (online/offline) updates. Must not be used together with `availability`.
+  required: false
+  type: string
+command_topic:
+  description: The MQTT topic to publish commands to change the switch state.
+  required: false
+  type: string
+icon:
+  description: Icon for the switch.
+  required: false
+  type: icon
+name:
+  description: The name to use when displaying this switch.
+  required: false
+  type: string
+  default: MQTT Switch
+payload_available:
+  description: The payload that represents the available state.
+  required: false
+  type: string
+  default: online
+payload_not_available:
+  description: The payload that represents the unavailable state.
+  required: false
+  type: string
+  default: offline
+payload_on:
+  description: The payload that represents `on` state. If specified, will be used for both comparing to the value in the `state_topic` (see `value_template` and `state_on`  for details) and sending as `on` command to the `command_topic`.
+  required: false
+  type: string
+  default: "ON"
+qos:
+  description: The maximum QoS level of the state topic. Default is 0 and will also be used to publishing messages.
+  required: false
+  type: integer
+  default: 0
+retain:
+  description: If the published message should have the retain flag on or not.
+  required: false
+  type: boolean
+  default: false
+unique_id:
+  description: An ID that uniquely identifies this switch device. If two switches have the same unique ID, Home Assistant will raise an exception.
+  required: false
+  type: string
+{% endconfiguration %}
+
+<div class='note warning'>
+
+Make sure that your topic matches exactly. `some-topic/` and `some-topic` are different topics.
+
+</div>
+
+## Examples
+
+In this section, you will find some real-life examples of how to use this sensor.
+
+### Full configuration
+
+The example below shows a full configuration for a switch.
+
+```yaml
+# Example configuration.yaml entry
+switch:
+  - platform: mqtt
+    unique_id: living_room_party_scene
+    name: "Living Room Party Scene"
+    command_topic: "home/living_room/party_scene/set"
+    availability:
+      - topic: "home/living_room/party_scene/available"
+    payload_on: "ON"
+    qos: 0
+    retain: true
+```
+
+### Use with a JSON Payload
+
+The example below shows a configuration using a JSON payload.
+
+```yaml
+# Example configuration.yaml entry
+scene:
+  - platform: mqtt
+    name: Living Room Blue Scene
+    unique_id: living_room_blue_scene
+    command_topic: "home/living_room/set"
+    payload_on: '{"activate_scene": "Blue Scene"}'
+```