doc: add schema

doc/schema.d2

@@ -0,0 +1,16 @@
+file: Input file {
+	shape: document
+	comm: Command lines { style.multiple: true }
+}
+comm: Command line
+
+rash {
+	parse: Parse
+	exec: Execute
+
+	parse -> exec
+}
+
+rash -> file: read
+file.comm -> rash.parse
+comm -> rash.parse