Tutorial: Improve Step 1

* Update minimum required version to 3.10
* Use VERSION argument to project command rather than separate variables
* Replace `endif(USE_MYMATH)` with  more modern `endif()`
* Simplify the call to 'configure_file()'
* Add comments to tutorial.cxx to use as anchors in documentation
* Remove CMakeLists and TutorialConfig.h.in files that users should
  create. Consequently, remove Step1 from CMake tests.

Help/guide/tutorial/Step1/tutorial.cxx

@@ -11,9 +11,11 @@ int main(int argc, char* argv[])
     return 1;
   }
 
-  double inputValue = atof(argv[1]);
+  // convert input to double
+  const double inputValue = atof(argv[1]);
 
-  double outputValue = sqrt(inputValue);
+  // calculate square root
+  const double outputValue = sqrt(inputValue);
   std::cout << "The square root of " << inputValue << " is " << outputValue
             << std::endl;
   return 0;