brightnessctl: add page (#6516)

pages/linux/brightnessctl.md

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+# brightnessctl
+
+> Utility for reading and controlling device brightness for GNU/Linux operating systems.
+> More information: <https://github.com/Hummer12007/brightnessctl>.
+
+- List devices with changeable brightness:
+
+`brightnessctl --list`
+
+- Print the current brightness of the display backlight:
+
+`brightnessctl get`
+
+- Set the brightness of the display backlight to a specified percentage within range:
+
+`brightnessctl set {{50%}}`
+
+- Increase brightness by a specified increment:
+
+`brightnessctl set {{+10%}}`
+
+- Decrease brightness by a specified decrement:
+
+`brightnessctl set {{-10%}}`