Add the start of a simple I/O guide

Apps/Guides/Simple_Input_Output/index.rst

+.. _simple_input_output_guide:
+
+Simple Input/Output
+===================
+
+The Librem 5 has a number of peripherals that allow applications to interact
+with users and the outside world in a simple way. This guide covers these I/O
+peripherals and describes how to use them.
+
+LEDs
+----
+
+Only one of the light-emitting diodes (LEDs) can be controlled by
+applications. This red diode is mapped to the ``/sys/class/leds/LED 1``
+directory in the file system and its state can be changed by writing
+to the ``/sys/class/leds/LED 1/brightness`` file, like this shell command::
+
+   echo 200 > /sys/class/leds/LED\ 1/brightness
+
+Using the default system configuration, writing a value of 0 to the file will
+switch the LED off. Writing a value from 1 to 255 will switch it on. If a
+different device tree is used with the kernel, this behavior can be adjusted.
+
+.. Perhaps link to issue https://source.puri.sm/Librem5/developer.puri.sm/issues/68

Apps/Guides/index.rst

@@ -12,6 +12,7 @@ to encounter. We aim to add more guides to this collection over time.
 
    Design/index
    Porting_GNOME_Applications/index
+   Simple_Input_Output/index
 
 We welcome suggestions for new guides. Please check the current
 `developer.puri.sm issues`_ for existing requests and submit a new request if