big improvements to tutorial, especially page 2 (matrix arithmetic).

add placeholders for some 'special topic' pages.

doc/examples/tut_arithmetic_dot_cross.cpp

@@ -0,0 +1,12 @@
+#include <iostream>
+#include <Eigen/Dense>
+
+using namespace Eigen;
+int main()
+{
+  Vector3d v(1,2,3);
+  Vector3d w(0,1,2);
+
+  std::cout << "Dot product: " << v.dot(w) << std::endl;
+  std::cout << "Cross product:\n" << v.cross(w) << std::endl;
+}