[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/Document.h

@@ -57,7 +57,11 @@ class Transaction;
 class StringHasher;
 using StringHasherRef = Base::Reference<StringHasher>;
 
-/// The document class
+/**
+ * @brief The document class
+ * @ingroup DocumentGroup
+ * @details For a more high-level discussion see the topic @ref DocumentGroup "Document".
+ */
 class AppExport Document: public App::PropertyContainer
 {
     PROPERTY_HEADER_WITH_OVERRIDE(App::Document);

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
 {

src/App/Expression.h

@@ -108,11 +108,14 @@ protected:
     int _changed{0};
 };
 
-/**
-  * Base class for expressions.
-  *
-  */
 
+/**
+  * @brief %Base class for expressions.
+  * @ingroup Expression
+  *
+  * @details For a high-level overview of the %Expression framework see topic
+  * @ref Expression "Expression Framework".
+  */
 class AppExport Expression : public Base::BaseClass {
     TYPESYSTEM_HEADER_WITH_OVERRIDE();
 

src/App/Property.h

@@ -44,14 +44,19 @@ namespace App
 class PropertyContainer;
 class ObjectIdentifier;
 
-/** Base class of all properties
- * This is the father of all properties. Properties are objects which are used
- * in the document tree to parametrize e.g. features and their graphical output.
+/**
+ * @brief %Base class of all properties
+ * @ingroup PropFrame
+ *
+ * @details This is the father of all properties. Properties are objects which that used
+ * in the document tree to parameterize e.g. features and their graphical output.
  * They are also used to gain access from the scripting facility.
- * /par
+ * @par
  * This abstract base class defines all methods shared by all
- * possible properties. It is also possible to define user properties
- * and use them in the framework...
+ * possible properties.  It is also possible to define user properties
+ * and use them in the framework.
+ *
+ * For a more high-level overview see topic @ref PropFrame "Property Framework".
  */
 class AppExport Property: public Base::Persistence
 {

src/App/core-app.dox

@@ -111,5 +111,7 @@
  *   - The DocumentObject classes
  *   - The Expression classes
  *   - The Property classes
+ *
+ * For a more high-level discussion see the topic @ref APP "App".
  */