doxygen: promote object management to a separate page

For "RT-Thread User Guide":
Take the "RT-Thread Kernel Object Model" sub-section out of
the "Kernel Basics" section and rename it to "Object Management".

Correspondingly, rename the "Kernel Object Management" section
in the "RT-Thread API Guide" to "Object Management".

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

include/rtthread.h

@@ -50,7 +50,7 @@ int entry(void);
 #endif
 
 /**
- * @addtogroup group_kernel_object
+ * @addtogroup group_object_management
  * @{
  */