asio: initial idf port of asio library without ssl

* Original commit: espressif/esp-idf@1ef13c524c

docs/en/api-reference/protocols/asio.rst

@@ -0,0 +1,32 @@
+ASIO port
+=========
+
+Overview
+--------
+Asio is a cross-platform C++ library, see https://think-async.com. It provides a consistent asynchronous model using a modern C++ approach.
+
+
+ASIO documentation
+^^^^^^^^^^^^^^^^^^
+Please refer to the original asio documentation at https://think-async.com/Asio/Documentation.
+Asio also comes with a number of examples which could be find under Documentation/Examples on that web site.
+
+Supported features
+^^^^^^^^^^^^^^^^^^
+ESP platform port currently supports only network asynchronous socket operations; does not support serial port and ssl.
+Internal asio settings for ESP include
+- EXCEPTIONS: Supported, choice in menuconfig
+- SIGNAL, SIGACTION: Not supported
+- EPOLL, EVENTFD: Not supported
+- TYPEID: Disabled by default, but supported in toolchain and asio (provided stdlib recompiled with -frtti)
+
+Application Example
+-------------------
+ESP examples are based on standard asio examples `examples/protocols/asio`:
+- udp_echo_server
+- tcp_echo_server
+- chat_client
+- chat_server
+Please refer to the specific example README.md for details
+
+