Assorted documentation fixes

* removed private flann documentation
* common tutorial images moved to doc/images
* grouping issues

modules/features2d/include/opencv2/features2d.hpp

@@ -80,6 +80,10 @@ This section describes approaches based on local 2D features and used to categor
     -   (Python) An example using the features2D framework to perform object categorization can be
         found at opencv_source_code/samples/python/find_obj.py
 
+    @defgroup feature2d_hal Hardware Acceleration Layer
+    @{
+        @defgroup features2d_hal_interface Interface
+    @}
   @}
  */
 

modules/features2d/include/opencv2/features2d/hal/interface.h

@@ -2,7 +2,7 @@
 #define OPENCV_FEATURE2D_HAL_INTERFACE_H
 
 #include "opencv2/core/cvdef.h"
-//! @addtogroup featrure2d_hal_interface
+//! @addtogroup features2d_hal_interface
 //! @{
 
 //! @name Fast feature detector types