doxygen: cleanup and re-org files

This patch contains two small changes, mainly to
prepare for the next step of organizing the modules
framework.

The first change: move the files related to doxygen
examples to `documentation/0.doxygen/example/`

The second change: delete `documentation/0.doxygen/mainpage.h`,
which is the mainpage of the old page. Now the new mainpage
is `documentation/INDEX.md`, so the duplicate can be
deleted.

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

documentation/0.doxygen/mainpage.h

@@ -1,49 +0,0 @@
-/*
- * This file is only used for doxygen document generation.
- */
-
-/**
- * @mainpage Introduction
- * @author RT-Thread Development Team
- * @version 1.2.0
- *
- * RT-Thread RTOS is an open source embedded real-time operating system and is
- * designed specifically for small memory footprint platforms. The real-time and
- * embedded characters are the most significant advantages of RT-Thread.
- *
- * - Real-Time Character
- *
- * RT-Thread has a real-time operating system kernel, with fully preempted
- * multi-thread scheduler, inter-thread communication with timing sensitivity
- * and transparent interrupt handling.
- *
- * - Embedded Character
- *
- * RT-Thread is suitable for embedded systems for small footprint characters.
- * The kernel is implemented as a simple C library. The simplest application
- * costs less than 1 Kbytes RAM on the ARM Cortex-M platform.
- *
- * @section kernel_arch RT-Thread Architecture
- *
- * RT-Thread system architecture is like:
- * @image html System_Arch.png "Figure 1: RT-Thread Architecture"
- *
- * @section kernel_service Kernel API
- *
- * The Kernel APIs are the core APIs of RT-Thread, which supports the following
- * features:
- * - Multi-thread management and scheduler
- * - Synchronization mechanisms, semaphore, recursive mutex and event set
- * - Inter-thread communication, mailbox and message queue
- * - Memory management, memory pool and dynamic heap memory management
- * - Asynchronous timer
- *
- * For more details, please refer to @ref group_Kernel
- *
- * @section system_init System Initialization
- *
- * Once RT-Thread operating system starts up, the facility in system must be initialized
- * firstly.
- *
- * For more details, please refer to @ref group_SystemInit
- */