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>

documentation/0.doxygen/1.core.h

@@ -14,8 +14,8 @@
  */
 
 /**
- * @defgroup group_kernel_object Kernel Object Management
- * @brief See @ref section_kernel_object_model
+ * @defgroup group_object_management Object Management
+ * @brief See @ref page_object_management
  *
  * The Kernel object system can access and manage all of the kernel objects.
  *