Add demo integration (#1346)

components/demo.rst

@@ -0,0 +1,29 @@
+Demo Integration
+================
+
+.. seo::
+    :description: Instructions for setting up the demo integration in ESPHome
+    :image: description.png
+
+The ``demo`` integration can be used for testing to generate sample instances of many
+different integrations (sensors, lights, ...)
+
+.. figure:: images/demo-ui.png
+    :align: center
+    :width: 70.0%
+
+.. code-block:: yaml
+
+    # Example configuration entry
+    demo:
+
+Configuration variables:
+------------------------
+
+This integration has no configuration variables
+
+See Also
+--------
+
+- :apiref:`demo/demo_sensor.h`
+- :ghedit:`Edit`

index.rst

@@ -464,6 +464,7 @@ Misc Components
     RTTTL Buzzer, components/rtttl, buzzer.jpg
     Prometheus, components/prometheus, prometheus.svg
     Grow Fingerprint Reader, components/fingerprint_grow, fingerprint.svg
+    Demo, components/demo, description.svg
 
 Additional Custom Components
 ----------------------------