gammastep: add page (#12196)

pages/common/gammastep.md

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+# Gammastep
+
+> Adjust the screen's color temperature according to the time of day.
+> More information: <https://gitlab.com/chinstrap/gammastep>.
+
+- Turn on Gammastep with a specific [t]emperature during the day (e.g. 5700k) and at night (e.g. 3600k):
+
+`gammastep -t {{5700}}:{{3600}}`
+
+- Turn on Gammastep with a manually specified custom [l]ocation:
+
+`gammastep -l {{latitude}}:{{longitude}}`
+
+- Turn on Gammastep with a specific screen [b]rightness during the day (e.g. 70%) and at night (e.g. 40%), with minimum brightness 10% and maximum brightness 100%:
+
+`gammastep -b {{0.7}}:{{0.4}}`
+
+- Turn on Gammastep with custom [g]amma levels (between 0 and 1):
+
+`gammastep -g {{red}}:{{green}}:{{blue}}`
+
+- Turn on Gammastep with a c[O]nstant unchanging color temperature:
+
+`gammastep -O {{temperature}}`
+
+- Reset temperature adjustments applied by Gammastep:
+
+`gammastep -x`