clean up usbh, rename doxygen folder to docs

docs/started_demo.md

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+# Demos #
+
+For simplicity and user's convenience, there are only 2 basic application demos which are *Device* and *Host* respectively. Each application demo, however, has a few projects, each for its supported RTOS. For instance, in addition to the *src* folder, you will also find in the /demo/device 
+
+- device\_os\_none for no RTOS
+- device\_freertos for freeRTOS
+- device\_cmsis_rtx for ARM CMSIS with RTX implemenation
+
+To be able to have the same application code running across RTOSes, the application make use of the "internal" **OSAL layer**. Thus this makes the application code a bit weird and over-complicated than it should be in some (many) cases. This is absolutely not necessary in product development. User can just use the native API function of supported RTOS or a state machine or blocking wait in case of none OS. For example, instead of the blinking task in application
+
+~~~{.c}
+OSAL_TASK_FUNCTION( led_blinking_task , p_task_para)
+{
+  OSAL_TASK_LOOP_BEGIN
+
+  static uint32_t led_on_mask = 0;
+
+  osal_task_delay(led_blink_interval_ms);
+
+  board_leds(led_on_mask, 1 - led_on_mask);
+  led_on_mask = 1 - led_on_mask; // toggle
+
+  OSAL_TASK_LOOP_END
+}
+~~~
+
+can be written in FreeRTOS's native API
+
+~~~{.c}
+void led_blinking_task( void * p_task_para )
+{
+  while(1)
+  {
+    static uint32_t led_on_mask = 0;
+
+    // FreeRTOS API's vTaskDelay is used in place of osal_task_delay. Note it takes input parameter in tick
+    vTaskDelay( (led_blink_interval_ms * CFG_TUSB_TICKS_HZ) / 1000);
+
+    board_leds(led_on_mask, 1 - led_on_mask);
+    led_on_mask = 1 - led_on_mask; // toggle
+  }
+}
+~~~
+
+
+[//]: # (\subpage md_demos_readme)
+[//]: # (\subpage md_demos_device_readme)
+[//]: # (\subpage md_demos_host_readme)