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/include/enum.h

@@ -14,15 +14,23 @@
  *
  * See
  * <a href="https://github.com/RT-Thread/rt-thread/blob/master/documentation/0.doxygen/example/include/enum.h">documentation/0.doxygen/example/include/enum.h</a>
- * for example.
+ * for code example.
+ *
+ * See
+ * <a href="./group__group__doxygen__example__enum.html">Doxygen Example of Enumeration</a>
+ * for html output.
  */
 
 /**
- * @addtogroup group_doxygen_example
+ * @defgroup group_doxygen_example_enum Doxygen Example of Enumeration
+ *
+ * @ingroup group_doxygen_example
+ *
+ * @brief Doxygen Example of Enumeration.
+ *
+ * @{
  */
 
-/** @{ */
-
 /**
  * @brief Brief description of this enumeration
  */