api version is documented in the docs

Closes #298

doc/qbk/main.qbk

@@ -164,6 +164,7 @@
 [import ../../test/doc/websocket_8_notes.cpp]
 
 [import ../../include/boost/beast/core/detect_ssl.hpp]
+[import ../../include/boost/beast/version.hpp]
 [import ../../test/beast/core/rate_policy.cpp]
 
 __new__ indicates a new or updated section in this version.
@@ -185,6 +186,7 @@ __new__ indicates an item that was recently added.
 [block'''</reference>''']
 
 [include release_notes.qbk]
+[include version.qbk]
 
 [include 01_intro/_intro.qbk]
 [include 02_examples/_examples.qbk]

doc/qbk/version.qbk

@@ -0,0 +1,21 @@
+ [/
+    Copyright (c) 2022 Klemens D. Morgenstern (klemens dot morgenstern at gmx dot net)
+
+    Distributed under the Boost Software License, Version 1.0. (See accompanying
+    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+    Official repository: https://github.com/boostorg/beast
+]
+
+[section Beast API Version]
+
+Beast maintains it's own API version, which is more fine-grained than the
+boost release.
+
+It can be obtained as a string from the `BOOST_BEAST_VERSION` macro 
+or a string through `BOOST_BEAST_VERSION_STRING`.
+
+[version]
+
+
+[endsect]