Various documentation improvements.

* Add short documentation for Array class
* Put all classes explicitly in Core module (where applicable)
* Section on Modules in Quick Reference Guide
* Put Page 7 after Page 6 in Contents :)

Eigen/src/Core/MatrixStorage.h

@@ -79,6 +79,7 @@ struct ei_matrix_array<T, 0, MatrixOptions, Alignment>
 /** \internal
   *
   * \class ei_matrix_storage
+  * \ingroup Core_Module
   *
   * \brief Stores the data of a matrix
   *