doc: sample: add examples of new extensible syntax

doc/sample/for-loop-1.mie

@@ -0,0 +1,14 @@
+func.func @reduce(%buffer: memref<1024*f32>, %lb: index, %ub: index, %step: index) -> f32 {
+	; Initial sum set to 0.
+	%sum.0 = arith.constant 0.0 : f32
+	; iter_args binds initial values to the loop's region arguments.
+	%sum = scf.for %iv = %lb to %ub step %step iter-args(%sum.iter = %sum.0) -> (f32) {
+		%t = memref.load %buffer[%iv] : memref<1024*f32>
+		%sum.next = arith.addf %sum.iter, %t : f32
+		; Yield current iteration sum to next iteration %sum.iter or to %sum
+		; if final iteration.
+		scf.yield %sum.next : f32
+	}
+
+	func.return %sum : f32
+}