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/DiagonalMatrix.h

@@ -87,6 +87,7 @@ void DiagonalBase<Derived>::evalTo(MatrixBase<DenseDerived> &other) const
 #endif
 
 /** \class DiagonalMatrix
+  * \ingroup Core_Module
   *
   * \brief Represents a diagonal matrix with its storage
   *
@@ -188,6 +189,7 @@ class DiagonalMatrix
 };
 
 /** \class DiagonalWrapper
+  * \ingroup Core_Module
   *
   * \brief Expression of a diagonal matrix
   *