brightness: add page (#2540)

pages/osx/brightness.md

@@ -0,0 +1,15 @@
+# brightness
+
+> Get and set the brightness level of all internal and certain external displays.
+
+- Show current brightness:
+
+`brightness -l`
+
+- Set the brightness to 100%::
+
+`brightness {{1}}`
+
+- Set the brightness to 50%::
+
+`brightness {{0.5}}`