Add button entity (#1114)

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

docs/core/entity/button.md

@@ -0,0 +1,40 @@
+---
+title: Button Entity
+sidebar_label: Button
+---
+
+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. It is, however, not suitable for implementing actual physical buttons; the sole purpose of a button entity is to provide a virtual button inside Home Assistant.
+
+A switch button entity is derived from the  [`homeassistant.components.button.ButtonEntity`](https://github.com/home-assistant/core/blob/dev/homeassistant/components/button/__init__.py),
+and can be helpful for controlling device features like (but not limited to):
+
+- Upgrading firmware
+- Reboot/Restart a device
+- Brew a cup of coffee
+- Reset something (like a counter, filter usage)
+
+If you want to represent something that can be turned on and off (and thus have an actual state), you should use a `switch` entity instead. If you want to integrate a real, physical, stateless button device in Home Assistant, you can do so by firing custom events. The entity button entity isn't suitable for these cases.
+
+
+
+## Properties
+
+As this integration is stateless, it doesn't provide any specific properties for itself.
+Other properties that are common to all entities such as `icon`, `name` etc are still applicable.
+
+## Methods
+
+### Press
+
+The press method can be used to trigger an action towards a device or service.
+It is called by Home Assistant when the user presses the button or the
+service to press the button has been called.
+
+```python
+class MyButton(ButtonEntity):
+    # Implement one of these methods.
+
+    def press(self) -> None:
+        """Handle the button press."""
+```