doxygen: re-org module groups (#10197)

Match the organization of modules (groups) with
that of user guide.

Preliminary arrangement. This patch is just a
framework arrangement. Details need to be continued.

P.S., in this patch, adding numerical prefixes to
the files in documentation/0.doxygen is to meet the
need of displaying sorting in HTML pages.

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

documentation/0.doxygen/2.components.h

@@ -0,0 +1,75 @@
+/*
+ * This file is only used for doxygen document generation.
+ */
+
+/**
+ * @defgroup group_kernel_components Components
+ *
+ * Components of RT-Thread, see @ref page_components for more details.
+ */
+
+/**
+ * @addtogroup group_kernel_components
+ * @{
+ */
+
+/**
+ * @defgroup group_DFS Device Virtual File System
+ *
+ * @brief DFS is a virtual file system in RT-Thread RTOS.
+ *
+ * The DFS (Device Virtual File System) is a vfs file system of RT-Thread RTOS,
+ * which is focused on embedded device. VFS is an abstraction layer on top of a
+ * more concrete file system. The purpose of a VFS is to allow client applications
+ * to access different types of concrete file systems in a uniform way.
+ *
+ * @image html dfs.png "Figure 4: Device Virtual File System Architecture"
+ *
+ * The DFS specifies an interface between the kernel and a concrete file system.
+ * Therefore, it is easy to add support for new file system types to the kernel
+ * simply by fulfilling the interface.
+ */
+
+/**
+ * @defgroup group_Device Device System
+ * @brief device I/O subsystem
+ *
+ * The Device System is designed as simple and minimum layer to help communication between
+ * applications and drivers.
+ *
+ * The Device System provide five interfaces to driver:
+ * - open, open a device
+ * - close, close a device
+ * - read, read some data from a device
+ * - write, write some data to a device
+ * - control, send some control command to a device
+ */
+
+/**
+ * @defgroup group_finsh finsh shell
+ *
+ * @brief finsh shell is a user command shell in RT-Thread RTOS.
+ *
+ * finsh shell is a user command shell in RT-Thread RTOS, which is a shell can
+ * accept C-expression like syntax in command. From finsh shell, user can access
+ * system area, such as memory, variables and function by input C-expression in
+ * command.
+ *
+ * @image html finsh.png "Figure 3: finsh shell architecture"
+ * There is a shell thread, which named as "tshell", in the finsh shell, it read
+ * user command from console device, and then invokes system function or access
+ * system variable to output result (by rt_kprintf).
+ */
+
+/**
+ * @defgroup group_Module Application Module
+ *
+ * @brief Application Module is a feature let user to execute application in RT-Thread RTOS.
+ *
+ * Application Module is implemented as dynamic object loader, but it can handle
+ * the dependences relationship between application and dynamic library, moreover,
+ * it also can handle the kernel object destroy and memory release issue when application
+ * (abnormally) exit.
+ */
+
+/**@}*/