DOc: move C++ to its own chapter

Documentation/Cookbook/rst/C++.rst

-.. _cpp-api:
-
-C++ API
-=======
-
-.. toctree::
-    :maxdepth: 1
-
-    C++/SystemOverview.rst
-    C++/Tutorial.rst
-    C++/UserGuide.rst
-    C++/Examples.rst
-
-    C++/Iterators.rst
-    C++/Filters.rst
-    C++/StreamingAndThreading.rst
-    C++/PersistentFilters.rst
-    C++/WriteAnApplication.rst
-    C++/AddingNewModules.rst

Documentation/Cookbook/rst/index.rst

@@ -56,11 +56,26 @@ Table of Contents
     Recipes
     Applications
 
+.. toctree::
+    :maxdepth: 2
+    :caption: C++ API
+
+    C++/SystemOverview.rst
+    C++/Tutorial.rst
+    C++/UserGuide.rst
+    C++/Examples.rst
+
+    C++/Iterators.rst
+    C++/Filters.rst
+    C++/StreamingAndThreading.rst
+    C++/PersistentFilters.rst
+    C++/WriteAnApplication.rst
+    C++/AddingNewModules.rst
+
 .. toctree::
     :maxdepth: 1
     :caption: Advanced use
 
-    C++
     EnvironmentVariables
     ExtendedFilenames
     CompilingOTBFromSource