Add scene entity developer documentation (#1850)

docs/core/entity/scene.md

@@ -0,0 +1,41 @@
+---
+title: Scene Entity
+sidebar_label: Scene
+--- 
+
+A scene entity is an entity that [can reproduce a wanted state](/docs/core/platform/reproduce_state/) for a group of entities. A scene entity can activate the scene towards a group of devices but remains stateless from the Home Assistant perspective.
+
+A scene entity is derived from [`homeassistant.components.scene.Scene`](https://github.com/home-assistant/core/blob/dev/homeassistant/components/scene/__init__.py).
+
+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.
+
+Scene entities can also [be created by the user via the Scene editor or YAML](https://www.home-assistant.io/integrations/scene).
+
+## 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` and `name` etc are still applicable.
+
+## Methods
+
+### Activate
+
+Activate the scene.
+
+```python
+class MySwitch(Scene):
+    # Implement one of these methods.
+
+    def activate(self, **kwargs: Any) -> None:
+        """Activate scene. Try to get entities into requested state."""
+
+    async def async_activate(self, **kwargs: Any) -> None:
+        """Activate scene. Try to get entities into requested state."""
+```
+
+The activate method can be used to activate the scene towards a device or service.
+It is called by Home Assistant when the user presses the scene `activate` button or when the `scene.turn_on` service is called to activate the scene.
+
+### Available device classes
+
+There are no specific device classes. The `device_class` attribute is not set on the scene entity.