doxygen: group examples in subpages

Doxygen examples are grouped on different pages so that you can
easily link to pages of related types in the documentation.
For example, macro examples have their own page, and function
examples have their own page, which correspond to their own code
examples, such as "macro.h" or "function.h".

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

documentation/0.doxygen/example/src/function.c

@@ -43,17 +43,27 @@
  *
  * See
  * <a href="https://github.com/RT-Thread/rt-thread/blob/master/documentation/0.doxygen/example/src/function.c">documentation/0.doxygen/example/src/function.c</a>
- * for example.
+ * for code example.
  *
- * <a href="https://github.com/RT-Thread/rt-thread/blob/master/documentation/0.doxygen/example/src/function.h">documentation/0.doxygen/example/src/function.h</a> is an example of the header file where we just declare the API without doxygen documentation.
+ * See
+ * <a href="./group__group__doxygen__example__function.html">Doxygen Example of Function</a>
+ * for html output.
+ *
+ * @note <a href="https://github.com/RT-Thread/rt-thread/blob/master/documentation/0.doxygen/example/src/function.h">documentation/0.doxygen/example/src/function.h</a>
+ * is just an example of the header file where we declare the API without
+ * doxygen documentation.
  */
 
 /**
- * @addtogroup group_doxygen_example
+ * @defgroup group_doxygen_example_function Doxygen Example of Function
+ *
+ * @ingroup group_doxygen_example
+ *
+ * @brief Doxygen Example of Function.
+ *
+ * @{
  */
 
-/** @{ */
-
 /**
  * @brief Brief description for the function
  *