Add man page. (#141)

* Add man page.

* CI: add scdoc

cage.1.scd

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+cage(1)
+
+# NAME
+
+cage - a Wayland kiosk compsitor
+
+# SYNOPSIS
+
+*cage* [-dhrv] [--] _application_ [application argument ...]
+
+# DESCRIPTION
+
+Cage runs a single, maximized application. Cage can run multiple applications,
+but only a single one is visible at any point in time. User interaction and
+activities outside the scope of the running application are prevented.
+
+# OPTIONS
+
+*-d*
+	Don't draw client side decorations when possible.
+
+*-h*
+	Show the help message.
+
+*-r*
+	Rotate the output 90 degrees clockwise. This can be specifed up to three
+	times, each resulting in an additional 90 degrees clockwise rotation.
+
+*-v*
+	Show the version number and exit.
+
+# ENVIRONMENT
+
+_DISPLAY_
+	If compiled with Xwayland support, this will be set to the name of the
+	X display used for Xwayland. Otherwise, probe the X11 backend.
+
+_WAYLAND_DISPLAY_
+	Specifies the name of the Wayland display that Cage is running on.
+
+_XCURSOR_PATH_
+	Directory where cursors are located.
+
+_XCURSOR_SIZE_
+	Specifies the configured cursor size.
+
+_XCURSOR_THEME_
+	Specifies the configured cursor theme.
+
+_XKB_DEFAULT_RULES_, _XKB_DEFAULT_MODEL_, _XKB_DEFAULT_LAYOUT_,
+_XKB_DEFAULT_VARIANT_, _XKB_DEFAULT_OPTIONS_
+	Configures the xkb keyboard settings. See *xkeyboard-config*(7).
+
+# SEE ALSO
+
+*xkeyboard-config(7)*
+
+# BUGS
+
+Report bugs at https://github.com/Hjdskes/cage
+
+# AUTHORS
+
+Jente Hidskes <dev@hjdskes.nl>