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/groups.h

@@ -0,0 +1,74 @@
+#ifndef _DOXYGEN_EXAMPLE_GROUPS_H_
+#define _DOXYGEN_EXAMPLE_GROUPS_H_
+
+/**
+ * @page page_howto_groups How to use groups in doxygen documentation.
+ *
+ * Doxygen has three mechanisms to group things together. For RT-Thread
+ * API documentation, we use 'topics' to create new page for each group.
+ * On HTML browser, the topics pages are shown under the "Modules" in the
+ * treeview on the left.
+ *
+ * To define a group, we use `@defgroup` command. The group name should be
+ * prefixed with a "group_", and the group name should be unique. We can
+ * define a group anywhere, not necessarily in the same file as the members
+ * of the group. Generally, we define a group in a header file.
+ *
+ * To make an entity (structure, function etc. or another group) a member of
+ * a speicific group, we can use `@ingroup` command and put the `@ingroup`
+ * command inside its documentation block.
+ *
+ * To avoid putting `@ingroup` commands in the documentation for each member
+ * you can use `@addtogroup` and group members together by the open marker
+ * `@{` before the group and the closing marker `@}` after the group.
+ *
+ * See
+ * <a href="https://github.com/RT-Thread/rt-thread/blob/master/documentation/0.doxygen/example/include/groups.h">documentation/0.doxygen/example/include/groups.h</a>
+ * for example.
+ *
+ * More information can be found in the Doxygen manual:
+ * <a href="https://www.doxygen.nl/manual/grouping.html">Grouping</a>.
+ */
+
+/**
+ * @defgroup group_doxygen_example_sub Sub Group of Doxygen Example
+ *
+ * All members of this group will be displayed in one HTML page.
+ *
+ * @ingroup group_doxygen_example
+ *
+ * @brief Define a sub group of Doxygen Example.
+ */
+
+/**
+ * @brief Brief description of this enumeration
+ *
+ * We use `@ingroup` to add this enum to the group_doxygen_example_sub separately.
+ *
+ * @ingroup group_doxygen_example_sub
+ */
+enum doxygen_example_enum_2
+{
+    V1, /**< description for value 1 */
+    V2, /**< description for value 2 */
+};
+
+// This entity is not a member of any group.
+#define DOXYGEN_EXAMPLE_CONST_E 300 /**< Description of macro const D */
+
+/**
+ * @addtogroup group_doxygen_example_sub
+ */
+
+/** @{ */
+
+/*
+ * DOXYGEN_EXAMPLE_CONST_C & DOXYGEN_EXAMPLE_CONST_D are close together, so
+ * we can use `@addtogroup`, `@{` and `@}` to group them together.
+ */
+#define DOXYGEN_EXAMPLE_CONST_C 100 /**< Description of macro const C */
+#define DOXYGEN_EXAMPLE_CONST_D 200 /**< Description of macro const D */
+
+/** @} */
+
+#endif /* _DOXYGEN_EXAMPLE_GROUPS_H_ */