[Doc] Improve topics within App

- The location of the documentation is improved (not in the cpp file
  anymore but in core-app.dox).  This prevents cluttering source with
  high-level overviews typical of topic documentation.
- The formatting has been made consistent.

src/App/Document.cpp

@@ -20,41 +20,6 @@
  *                                                                         *
  ***************************************************************************/
 
-
-/*!
-\defgroup Document Document
-\ingroup APP
-\brief The Base class of the FreeCAD Document
-
-This (besides the App::Application class) is the most important class in FreeCAD.
-It contains all the data of the opened, saved, or newly created FreeCAD Document.
-The App::Document manages the Undo and Redo mechanism and the linking of documents.
-
-\namespace App \class App::Document
-This is besides the Application class the most important class in FreeCAD
-It contains all the data of the opened, saved or newly created FreeCAD Document.
-The Document manage the Undo and Redo mechanism and the linking of documents.
-
-Note: the documents are not free objects. They are completely handled by the
-App::Application. Only the Application can Open or destroy a document.
-
-\section Exception Exception handling
-As the document is the main data structure of FreeCAD we have to take a close
-look at how Exceptions affect the integrity of the App::Document.
-
-\section UndoRedo Undo Redo an Transactions
-Undo Redo handling is one of the major mechanism of a document in terms of
-user friendliness and speed (no one will wait for Undo too long).
-
-\section Dependency Graph and dependency handling
-The FreeCAD document handles the dependencies of its DocumentObjects with
-an adjacence list. This gives the opportunity to calculate the shortest
-recompute path. Also, it enables more complicated dependencies beyond trees.
-
-@see App::Application
-@see App::DocumentObject
-*/
-
 #include "PreCompiled.h"
 
 #ifndef _PreComp_