docs: use scdoc instead of roff for man pages

doc/river.1.scd

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+RIVER(1) "github.com/ifreund/river" "General Commands Manual"
+# NAME
+
+river - dynamic tiling Wayland compositor
+
+# SYNOPSIS
+
+*river* [*-c* _shell_command_]
+
+# DESCRIPTION
+
+*river* is a dynamic tiling Wayland compositor inspired by dwm and
+bspwm based on wlroots and written in Zig.
+
+# OPTIONS
+
+*-c* _shell_command_
+	Run a shell command or give the path to a script that will be run
+	before river gets started. This is where you define keymaps using
+	*riverctl*(1) and autostart programs. Make sure the script is
+	executable.
+
+# CONFIGURATION
+
+View border width, gap sizes, and programs which should float are
+configured in _Config.zig_. Make your changes and recompile.
+
+Experimental XWayland support can be enabled on compile-time with the
+_-Dxwayland=true_ flag.
+
+Keyboard layout and other XKB options can be set in _Keyboard.zig_.
+
+# SEE ALSO
+
+*riverctl*(1), *bspwm*(1), *dwm*(1)