showkey: add page (#12550)

* showkey v1

* added sudo

* showkey: add page

* showkey: add page

Fixed build test errors

* Apply suggestions from code review

- Imperative mood
- ctrl +d
- added long options
- changed key-caps in accordance to style guide

Co-authored-by: Vitor Henrique <87824454+vitorhcl@users.noreply.github.com>

* Update pages/linux/showkey.md

Co-authored-by: Vitor Henrique <87824454+vitorhcl@users.noreply.github.com>

---------

Co-authored-by: Vitor Henrique <87824454+vitorhcl@users.noreply.github.com>

pages/linux/showkey.md

@@ -0,0 +1,24 @@
+# showkey
+
+> Display the keycode of pressed keys on the keyboard, helpful for debugging keyboard-related issues and key remapping.
+> More information: <https://manned.org/showkey>.
+
+- View keycodes in decimal:
+
+`sudo showkey`
+
+- Display [s]cancodes in hexadecimal:
+
+`sudo showkey {{-s|--scancodes}}`
+
+- Display [k]eycodes in decimal (default):
+
+`sudo showkey {{-k|--keycodes}}`
+
+- Display keycodes in [a]SCII, decimal, and hexadecimal:
+
+`sudo showkey {{-a|--ascii}}`
+
+- Exit the program:
+
+`Ctrl + d`