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/BandMatrix.h

@@ -27,6 +27,7 @@
 
 /**
   * \class BandMatrix
+  * \ingroup Core_Module
   *
   * \brief Represents a rectangular matrix with a banded storage
   *
@@ -205,6 +206,7 @@ class BandMatrix : public EigenBase<BandMatrix<_Scalar,Rows,Cols,Supers,Subs,Opt
 
 /**
   * \class TridiagonalMatrix
+  * \ingroup Core_Module
   *
   * \brief Represents a tridiagonal matrix
   *