[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/DocumentObject.cpp

@@ -55,10 +55,6 @@ FC_LOG_LEVEL_INIT("App", true, true)
 
 using namespace App;
 
-/** \defgroup DocObject Document Object
-    \ingroup APP
-    \brief Base class of all objects handled in the Document
-*/
 
 PROPERTY_SOURCE(App::DocumentObject, App::TransactionalObject)