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/struct.h

@@ -0,0 +1,78 @@
+#ifndef _DOXYGEN_EXAMPLE_STRUCT_H_
+#define _DOXYGEN_EXAMPLE_STRUCT_H_
+
+/**
+ * @page page_howto_struct How to write doxygen documentation for structure.
+ *
+ * We recommend the following approach:
+ *
+ * A comment block before the structure definition is recommended to
+ * describe the general information of the structure type. In the
+ * comment block, a `@brief` is required, other commands (such as `@note`)
+ * are optional.
+ *
+ * If you feel that the description of `@brief` is not enough, you
+ * can add a detailed description part, which is also optional.
+ *
+ * Put member comments inside the structure definition and after every member.
+ *
+ * See
+ * <a href="https://github.com/RT-Thread/rt-thread/blob/master/documentation/0.doxygen/example/include/struct.h">documentation/0.doxygen/example/include/struct.h</a>
+ * for example.
+ */
+
+/**
+ * @addtogroup group_doxygen_example
+ */
+
+/** @{ */
+
+/**
+ * @brief Brief description this structure
+ *
+ * Detailed description starts here, one line or multiple lines.
+ * Blah blah blah...
+ *
+ * @note This is a note for this structure, blah blah blah...
+ */
+struct dogygen_example_struct
+{
+    int m1; /**< Some documentation for member 'm1'...
+             * Multiple lines ... Note the "multi-line" here refers
+             * to the code. Whether it is displayed in multiple lines
+             * on the specific HTML page depends on the browser rendering.
+             *
+             * @note We can also embed note for member 'm1'...
+             */
+    int m2; /**< Some documentation for member 'm2'... */
+    int m3; /**< Some documentation for member 'm3'... */
+    int m4; /**< Some documentation for member 'm4'... */
+    int m5; /**< Some documentation for member 'm5'... */
+};
+
+/**
+ * @brief Brief description this structure
+ *
+ * Detailed description starts here, one line or multiple lines.
+ * Blah blah blah...
+ *
+ * @note This is a note for this structure, blah blah blah...
+ */
+struct dogygen_example_struct_another
+{
+    int m1; /**< Some documentation for member 'm1'...
+             * Multiple lines ... Note the "multi-line" here refers
+             * to the code. Whether it is displayed in multiple lines
+             * on the specific HTML page depends on the browser rendering.
+             *
+             * @note We can also embed note for member 'm1'...
+             */
+    int m2; /**< Some documentation for member 'm2'... */
+    int m3; /**< Some documentation for member 'm3'... */
+    int m4; /**< Some documentation for member 'm4'... */
+    int m5; /**< Some documentation for member 'm5'... */
+};
+
+/** @} */
+
+#endif /* _DOXYGEN_EXAMPLE_STRUCT_H_ */