doxygen: add documentation for doxygen

Documentation is provided to clarify how to write
doxygen documentation for RT-Thread. This document
is also integrated as part of RT-Thread doxygen
documentation.

An example is also provided.

The original README.md is removed and integrated into
this document.

Updated github actions for doxygen too.

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

documentation/0.doxygen/example/include/macro.h

@@ -0,0 +1,44 @@
+#ifndef _DOXYGEN_EXAMPLE_MACRO_H_
+#define _DOXYGEN_EXAMPLE_MACRO_H_
+
+/**
+ * @page page_howto_macro How to write doxygen documentation for macro.
+ *
+ * There are two typical types of macro definitions.
+ *
+ * - One is to define constant macros. For this type of macro, we
+ *   recommend putting documentation after members. See `DOXYGEN_EXAMPLE_CONST_A`
+ *   and `DOXYGEN_EXAMPLE_CONST_B` in
+ *   <a href="https://github.com/RT-Thread/rt-thread/blob/master/documentation/0.doxygen/example/include/macro.h">documentation/0.doxygen/example/include/macro.h</a>
+ *   for exmaple.
+ *
+ * - The other is to define macros with parameters. For this type of
+ *   macro, we recommend using a method similar to documenting for
+ *   functions, that is, writing comment block before the macro definition.
+ *   More details please see @ref page_howto_function
+ *   See `DOXYGEN_EXAMPLE_ABS` in
+ *   <a href="https://github.com/RT-Thread/rt-thread/blob/master/documentation/0.doxygen/example/include/macro.h">documentation/0.doxygen/example/include/macro.h</a>
+ *   for example.
+ */
+
+/**
+ * @addtogroup group_doxygen_example
+ */
+
+/** @{ */
+
+#define DOXYGEN_EXAMPLE_CONST_A 100 /**< Description of macro const A */
+#define DOXYGEN_EXAMPLE_CONST_B 200 /**< Description of macro const B */
+
+/**
+ * @brief Computes the absolute value of its argument @a x.
+ *
+ * @param[in] x input value.
+ *
+ * @return absolute value of @a x.
+ */
+#define DOXYGEN_EXAMPLE_ABS(x) (((x)>0)?(x):-(x))
+
+/** @} */
+
+#endif