xbacklight: add page (#3273)

pages/linux/xbacklight.md

@@ -0,0 +1,24 @@
+# xbacklight
+
+> Utility to adjust backlight brightness using the RandR extension.
+> More information: <https://gitlab.freedesktop.org/xorg/app/xbacklight>.
+
+- Get the current screen brightness as a percentage:
+
+`xbacklight`
+
+- Set the screen brightness to 40%:
+
+`xbacklight -set {{40}}`
+
+- Increase current brightness by 25%:
+
+`xbacklight -inc {{25}}`
+
+- Decrease current brightness by 75%:
+
+`xbacklight -dec {{75}}`
+
+- Increase backlight to 100%, over 60 seconds (value given in ms), using 60 steps:
+
+`xbacklight -set {{100}} -time {{60000}} -steps {{60}}`