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/enum.h

@@ -0,0 +1,37 @@
+#ifndef _DOXYGEN_EXAMPLE_ENUM_H_
+#define _DOXYGEN_EXAMPLE_ENUM_H_
+
+/**
+ * @page page_howto_enum How to write doxygen documentation for enumeration.
+ *
+ * A comment block before the enumeration definition is recommended to
+ * describe the general information of the enumeration type. In the
+ * comment block, a `@brief` is required, other commands (such as `@note`)
+ * are optional.
+ *
+ * To describe the values of the enumeration, document is recommended
+ * to be put after each value.
+ *
+ * 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.
+ */
+
+/**
+ * @addtogroup group_doxygen_example
+ */
+
+/** @{ */
+
+/**
+ * @brief Brief description of this enumeration
+ */
+enum doxygen_example_enum
+{
+    V1, /**< description for value 1 */
+    V2, /**< description for value 2 */
+};
+
+/** @} */
+
+#endif /* _DOXYGEN_EXAMPLE_ENUM_H_ */