docs: call_v_from_c example (#15844)

examples/call_v_from_c/v_test_math.v

@@ -0,0 +1,13 @@
+module test_math
+
+import math
+
+[export: 'square']
+fn calculate_square(i int) int {
+	return i * i
+}
+
+[export: 'sqrt_of_sum_of_squares']
+fn calculate_sqrt_of_sum_of_squares(x f64, y f64) f64 {
+	return math.sqrt(x * x + y * y)
+}