Add button entity (#20105)

Co-authored-by: Martin Hjelmare <marhje52@gmail.com>

source/_integrations/button.markdown

@@ -0,0 +1,54 @@
+---
+title: Button
+description: Instructions on how to set up your button with Home Assistant.
+ha_category:
+  - Button
+ha_release: 2021.12
+ha_quality_scale: internal
+ha_domain: button
+ha_codeowners:
+  - '@home-assistant/core'
+---
+
+A button entity is an entity that can fire an event / trigger an action towards
+a device or service but remains stateless from the Home Assistant perspective.
+
+It can be compared to a real live momentary switch, push-button, or some other
+form of a stateless switch.
+
+The button entities cannot be implemented manually, but can be provided by
+other integrations.
+
+## The state of a button
+
+The button entity is stateless, as in, it cannot have a state like the `on` or
+`off` state that, for example, a normal switch entity has.
+
+Every button entity does keep track of the timestamp of when the last time
+the button entity has been pressed in the Home Assistant UI or pressed via
+a service call.
+
+Because the state of a button entity in Home Assistant is a timestamp, it
+means we can use it in our automations. For example:
+
+```yaml
+trigger:
+  - platform: state
+    entity_id: button.my_button
+action:
+  - service: notify.frenck
+    data:
+      message: "My button has been pressed!"
+```
+
+## Services
+
+The button entities exposes a single service: {% my developer_call_service service="button.press" %}
+
+This service can be called to trigger a button press for that entity.
+
+```yaml
+- service: button.press
+  target:
+    entity_id: button.my_button
+```