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/TriangularMatrix.h

@@ -29,6 +29,7 @@
 /** \internal
   *
   * \class TriangularBase
+  * \ingroup Core_Module
   *
   * \brief Base class for triangular part in a matrix
   */
@@ -112,6 +113,7 @@ template<typename Derived> class TriangularBase : public EigenBase<Derived>
 };
 
 /** \class TriangularView
+  * \ingroup Core_Module
   *
   * \brief Base class for triangular part in a matrix
   *