gummy: add page (#9900)

pages/linux/gummy.md

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+# gummy
+
+> Screen brightness/temperature manager for Linux/X11.
+> More information: <https://github.com/Fushko/gummy>.
+
+- Set the screen temperature to 3000K:
+
+`gummy --temperature {{3000}}`
+
+- Set the screen backlight to 50%:
+
+`gummy --backlight {{50}}`
+
+- Set the screen pixel brightness to 45%:
+
+`gummy --brightness {{45}}`
+
+- Increase current screen pixel brightness by 10%:
+
+`gummy --brightness {{+10}}`
+
+- Decrease current screen pixel brightness by 10%:
+
+`gummy --brightness {{-10}}`
+
+- Set the temperature and pixel brightness for the second screen:
+
+`gummy --screen {{1}} --temperature {{3800}} --brightness {{65}}`