[Doc] Add links between topics and relevant items

The documentation has topics, for example topic App which contains
namespace App or topic Document that contains class Document.  This
commit makes sure that the topic has a link to the relevant class or
namespace, whereas the relevant class or namespace refers back to the
topic for a more high-level overview.

src/App/DocumentObject.h

@@ -101,7 +101,10 @@ public:
 };
 
 
-/** Base class of all Classes handled in the Document
+/**
+ * @brief %Base class of all objects handled in the @ref App::Document "Document".
+ * @ingroup DocObject
+ * @details For a more high-level overview see topic @ref DocObject "Document Object".
  */
 class AppExport DocumentObject: public App::TransactionalObject
 {