Add doc for duty cycle output component (#427)

* Add doc for duty cycle output component

* add to index

* png -> svg

* .

* See also

* duty_cycle -> slow_pwm

* .

* .

* wrap long line

* Update components/output/slow_pwm.rst

Co-Authored-By: Otto Winter <otto@otto-winter.com>

components/output/slow_pwm.rst

@@ -0,0 +1,49 @@
+Slow PWM Output
+===============
+
+.. seo::
+    :description: Instructions for setting up slow pwm outputs for GPIO pins.
+    :image: pwm.png
+
+Similar to PWM, the Slow PWM Output platform allows you to control GPIO pins by
+pulsing them on/off over a longer time period. It could be used to control a
+heating element through a relay where a fast PWM update cycle would not be appropriate.
+
+.. note::
+
+    This is for **slow** PWM output. For fast-switching PWM outputs (for example,
+    lights), see these outputs:
+
+    - ESP32: :doc:`ledc`
+    - ESP8266: :doc:`esp8266_pwm`
+
+.. code-block:: yaml
+
+    # Example configuration entry
+    output:
+      - platform: slow_pwm
+        pin: D1
+        id: my_slow_pwm
+        period: 15s
+
+
+Configuration variables:
+------------------------
+
+- **pin** (**Required**, :ref:`Pin Schema <config-pin_schema>`): The pin to pulse.
+- **id** (**Required**, :ref:`config-id`): The id to use for this output component.
+- **period** (**Required**, :ref:`config-time`): The duration of each cycle. (i.e. a 10s
+  period at 50% duty would result in the pin being turned on for 5s, then off for 5s)
+- All other options from :ref:`Output <config-output>`.
+
+See Also
+--------
+
+- :doc:`/components/output/index`
+- :doc:`/components/output/esp8266_pwm`
+- :doc:`/components/output/ledc`
+- :doc:`/components/light/monochromatic`
+- :doc:`/components/fan/speed`
+- :doc:`/components/power_supply`
+- :apiref:`slow_pwm/slow_pwm_output.h`
+- :ghedit:`Edit`

index.rst

@@ -194,6 +194,7 @@ Output Components
 
     Output Core, components/output/index, folder-open.svg
     ESP8266 Software PWM, components/output/esp8266_pwm, pwm.png
+    Slow PWM, components/output/slow_pwm, pwm.png
     GPIO Output, components/output/gpio, pin.svg
     ESP32 LEDC, components/output/ledc, pwm.png
     PCA9685, components/output/pca9685, pca9685.jpg