MQTT button config to integration key (#22816)

source/_integrations/button.mqtt.markdown

@@ -14,11 +14,30 @@ The `mqtt` button platform lets you send an MQTT message when the button is pres
 
 ```yaml
 # Example configuration.yaml entry
+mqtt:
+  button:
+    - command_topic: "home/bedroom/switch1/reboot"
+```
+
+<a id='new_format'></a>
+
+{% details "Previous configuration format" %}
+
+The configuration format of manual configured MQTT items has changed.
+The old format that places configurations under the `button` platform key
+should no longer be used and is deprecated.
+
+The above example shows the new and modern way,
+this is the previous/old example:
+
+```yaml
 button:
   - platform: mqtt
     command_topic: "home/bedroom/switch1/reboot"
 ```
 
+{% enddetails %}
+
 {% configuration %}
 availability:
   description: A list of MQTT topics subscribed to receive availability (online/offline) updates. Must not be used together with `availability_topic`.
@@ -193,16 +212,16 @@ The example below shows a full configuration for a button.
 
 ```yaml
 # Example configuration.yaml entry
-button:
-  - platform: mqtt
-    unique_id: bedroom_switch_reboot_btn
-    name: "Restart Bedroom Switch"
-    command_topic: "home/bedroom/switch1/commands"
-    payload_press: "restart"
-    availability:
-      - topic: "home/bedroom/switch1/available"
-    qos: 0
-    retain: false
-    entity_category: "config"
-    device_class: "restart"
+mqtt:
+  button:
+    - unique_id: bedroom_switch_reboot_btn
+      name: "Restart Bedroom Switch"
+      command_topic: "home/bedroom/switch1/commands"
+      payload_press: "restart"
+      availability:
+        - topic: "home/bedroom/switch1/available"
+      qos: 0
+      retain: false
+      entity_category: "config"
+      device_class: "restart"
 ```