Add docs for SDL binary sensor (#4644)

Co-authored-by: clydebarrow <2366188+clydebarrow@users.noreply.github.com>

components/binary_sensor/sdl.rst

@@ -0,0 +1,29 @@
+SDL Binary Sensor
+=================
+
+.. seo::
+    :description: Instructions for setting up an SDL keyboard binary sensor.
+
+The ``sdl`` binary sensor platform creates a binary sensor from keyboard presses on the host platform. 
+The sensor will be true when the key is pressed.
+
+Configuration variables:
+------------------------
+
+- **key** (**Required**): The ID of an `SDL key <https://www.libsdl.org/release/SDL-1.2.15/docs/html/sdlkey.html>`_.
+- All other variables from :ref:`Binary Sensor <config-binary_sensor>`.
+
+Example:
+
+.. code-block:: yaml
+
+    binary_sensor:
+      - platform: sdl
+        id: key_id
+        key: SDLK_a
+
+See Also
+--------
+- :doc:`Host Platform </components/host>`
+- :ref:`SDL display <sdl>`
+- :ghedit:`Edit`

components/display/sdl.rst

@@ -100,6 +100,7 @@ See Also
 --------
 
 - :ref:`SDL touchscreen <sdl_touchscreen>`
+- :doc:`SDL binary sensor </components/binary_sensor/sdl>`
 - :doc:`index`
 - :apiref:`sdl/sdl_esphome.h`
 - :ghedit:`Edit`

index.rst

@@ -652,6 +652,7 @@ Core
     Home Assistant, components/binary_sensor/homeassistant, home-assistant.svg, dark-invert
     Status, components/binary_sensor/status, server-network.svg, dark-invert
     Switch, components/binary_sensor/switch, electric-switch.svg, dark-invert
+    Host SDL2, components/binary_sensor/sdl, sdl.png
 
 Capacitive Touch
 ****************