Help/guide/tutorial: Adopt tutorial code

Help/guide/tutorial/Step1/tutorial.cxx

@@ -0,0 +1,20 @@
+// A simple program that computes the square root of a number
+#include <cmath>
+#include <cstdlib>
+#include <iostream>
+#include <string>
+
+int main(int argc, char* argv[])
+{
+  if (argc < 2) {
+    std::cout << "Usage: " << argv[0] << " number" << std::endl;
+    return 1;
+  }
+
+  double inputValue = atof(argv[1]);
+
+  double outputValue = sqrt(inputValue);
+  std::cout << "The square root of " << inputValue << " is " << outputValue
+            << std::endl;
+  return 0;
+}