fs(3lua): init man pages

Initialise man pages for the fs module.

man/fs/fs.exists.3lua.scd

@@ -0,0 +1,21 @@
+FS.EXISTS(3lua) "3lua" "Callisto manual pages"
+
+# NAME
+*fs.exists* - check existence of a path in the file system
+
+# SYNOPSIS
+*fs.exists*(_path_: *string*)
+
+# DESCRIPTION
+*fs.exists* returns whether the pathname _path_ exists
+in the file system.
+
+# EXAMPLES
+Check whether _tel_ exists:
+
+	if not fs.exists("tel") then
+		print("tel does not exist")
+	end
+
+# SEE ALSO
+callisto(3lua), fs(3lua), fs.isfile(3lua)