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/VectorBlock.h

@@ -27,6 +27,7 @@
 #define EIGEN_VECTORBLOCK_H
 
 /** \class VectorBlock
+  * \ingroup Core_Module
   *
   * \brief Expression of a fixed-size or dynamic-size sub-vector
   *