screenkey: add page (#5999)

Co-authored-by: CleanMachine1 <78213164+CleanMachine1@users.noreply.github.com>
Co-authored-by: bl-ue <54780737+bl-ue@users.noreply.github.com>
Co-authored-by: marchersimon <50295997+marchersimon@users.noreply.github.com>

pages/linux/screenkey.md

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+# screenkey
+
+> A screencast tool to display keys pressed.
+> More information: <https://www.thregr.org/~wavexx/software/screenkey/>.
+
+- Display keys which are currently being pressed on the screen:
+
+`screenkey`
+
+- Display keys and mouse buttons which are currently being pressed on the screen:
+
+`screenkey --mouse`
+
+- Launch the settings menu of screenkey:
+
+`screenkey --show-settings`
+
+- Launch screenkey at a specific position:
+
+`screenkey --position {{top|center|bottom|fixed}}`
+
+- Change the format of the key modifiers displayed on screen:
+
+`screenkey --mods-mode {{normal|emacs|mac|win|tux}}`
+
+- Change the appearance of screenkey:
+
+`screenkey --bg-color "{{#a1b2c3}}" --font {{Hack}} --font-color {{yellow}} --opacity {{0.8}}`
+
+- Drag and select a window on screen to display screenkey:
+
+`screenkey --position fixed --geometry {{$(slop -n -f '%g')}}`