Add date entity developer docs (#1533)

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

docs/core/entity/date.md

@@ -0,0 +1,36 @@
+---
+title: Date Entity
+sidebar_label: Date
+---
+
+A `date` is an entity that allows the user to input a date to an integration. Derive entity platforms from [`homeassistant.components.date.DateEntity`](https://github.com/home-assistant/core/blob/dev/homeassistant/components/date/__init__.py)
+
+## Properties
+
+:::tip
+Properties should always only return information from memory and not do I/O (like network requests). Implement `update()` or `async_update()` to fetch data.
+:::
+
+| Name | Type | Default | Description
+| ---- | ---- | ------- | -----------
+| native_value | date | **Required** | The value of the date.
+
+Other properties that are common to all entities such as `icon`, `name` etc are also applicable.
+
+## Methods
+
+### Set value
+
+Called when the user or an automation wants to update the value.
+
+```python
+class MyDate(DateEntity):
+    # Implement one of these methods.
+
+    def set_value(self, value: date) -> None:
+        """Update the current value."""
+
+    async def async_set_value(self, value: date) -> None:
+        """Update the current value."""
+
+```