Add backlight information (#78)

Co-authored-by: Nathaniel van Diepen <[email protected]>

src/devel/device/display.rst

@@ -21,3 +21,36 @@ reMarkable 2 Framebuffer
 ========================
 
 https://github.com/ddvk/remarkable2-framebuffer
+
+
+Changing the Device Backlight
+=============================
+
+The reMarkable Paper Pro has a backlight that can be adjusted to make the screen easier to read in different lighting conditions.
+
+Adjusting in the UI
+-------------------
+Xochitl provides 5 presets of backlight brightness, which can be accessed by swiping down from the top-right of the screen to open the `quick settings menu <https://support.remarkable.com/s/article/Quick-settings>`_
+
+The Xochitl backlight presets are as follows:
+
+#. 260 (12%)
+#. 694 (32%)
+#. 1040 (50%)
+#. 1387 (67%)
+#. 1734 (84%)
+
+Adjusting programmatically
+--------------------------
+
+:raw-html:`<div class="warning">⚠️ Warning. ⚠️<br>`
+Setting the brightness outside of the range you can select in xochitl will make the battery drain faster, and may have unintended side effects that could damage the device.
+:raw-html:`</div>`
+
+The backlight can also be adjusted programmatically by writing to the `/sys/class/backlight/backlight/brightness` file.
+The value written to this file should be between 0 and 2047, where 0 is completely off and 2047 is the maximum brightness.
+
+.. code-block:: shell
+
+  echo 1734 > /sys/class/backlight/backlight/brightness
+