docs: add rivertile man page

doc/rivertile.1.scd

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+RIVERTILE(1) "github.com/ifreund/river" "General Commands Manual"
+
+# NAME
+
+rivertile - tiled layout generator for river
+
+# SYNOPSIS
+
+*rivertile* *left*|*right*|*top*|*bottom* [args passed by river]
+
+# DESCRIPTION
+
+*rivertile* is a layout generator for river. It produces tiled layouts with
+split master/secondary stacks in four configurable orientations.
+
+# OPTIONS
+
+*left*
+	Place the master stack on the left side of the output.
+
+*right*
+	Place the master stack on the right side of the output.
+
+*top*
+	Place the master stack at the top of the output.
+
+*bottom*
+	Place the master stack at the bottom of the output.
+
+# EXAMPLE
+
+Set river's layout to *rivertile*'s *left* layout using riverctl
+
+	riverctl layout rivertile left
+
+# SEE ALSO
+
+*river-layouts*(7), *river*(1), *riverctl*(1)