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/src/function.c

@@ -0,0 +1,95 @@
+/**
+ * @page page_howto_function How to write doxygen documentation for function.
+ *
+ * Function comments can be placed in the header file (before the
+ * function declaration) OR in the source file (before the function
+ * definition).
+ *
+ * The advantage of placing it in the header file is that we generally
+ * think that the header file is the place to declare the API, but the
+ * problem is that when a module has many API extern functions, if the
+ * comments are placed in the header file, the header file will be very
+ * long. You can imagine that for a module with many APIs and many
+ * comments, the header file will be full of large green comments, and
+ * the function declaration part is mixed in the middle and difficult
+ * to distinguish. And if you want to fully understand which extern
+ * functions this module exports, you need to scroll a long file for a
+ * long time to get a general idea. Especially for RTT, see `include/rtthread.h`
+ * as an example.
+ *
+ * Putting the comment in the source file can avoid the above problems.
+ * For developers, it is also convenient to read the comments together with
+ * the code implementation. The disadvantage is that it would be different
+ * from the function side, from other types, such as structures, i.e. the API
+ * comments of functions need to be read in the source file instead of directly
+ * in the header file.
+ *
+ * Comprehensive consideration can be combined with the actual situation.
+ * For example, if there are too many API functions in a header file, it is
+ * recommended to put the function comments in the source file.
+ *
+ * So, it is **strongly recommended** to put comments in the source file when
+ * writing new functions or annotating functions.
+ *
+ * To documenting for functions, a comment block before the function
+ * declaraion/definition is recommended to describe the general information
+ * of the function. In the comment block, a `@brief` is required, a `@return`
+ * is also required no matter if the function return values or not. `@param` is
+ * required if any, and if it is provided, direction [in]/[out]/[in,out] should
+ * be provide together. 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.
+ *
+ * See
+ * <a href="https://github.com/RT-Thread/rt-thread/blob/master/documentation/0.doxygen/example/src/function.c">documentation/0.doxygen/example/src/function.c</a>
+ * for example.
+ *
+ * <a href="https://github.com/RT-Thread/rt-thread/blob/master/documentation/0.doxygen/example/src/function.h">documentation/0.doxygen/example/src/function.h</a> is an example of the header file where we just declare the API without doxygen documentation.
+ */
+
+/**
+ * @addtogroup group_doxygen_example
+ */
+
+/** @{ */
+
+/**
+ * @brief Brief description for the function
+ *
+ * Detailed description starts here, one line or multiple lines.
+ * Blah blah blah...
+ *
+ * @param[in] a Description of param a
+ *
+ * @param[in] b Description of param b
+ *
+ * @return void
+ *
+ * @note This is a note for this structure, blah blah blah...
+ */
+void doxygen_example_func_foo(int a, int b)
+{
+    return;
+}
+
+/**
+ * @brief Brief description for the function
+ *
+ * Detailed description starts here, one line or multiple lines.
+ * Blah blah blah...
+ *
+ * @param[in] a Description of param a
+ *
+ * @param[out] b Description of param b
+ *
+ * @return the return value, 0 for success, -1 for failure
+ *
+ * @note This is a note for this structure, blah blah blah...
+ */
+int doxygen_example_func_bar(int a, int* b)
+{
+    return 0;
+}
+
+/** @} */