Simplify document generation and replace Async.MQTT5 with Boost.MQTT5 in docs

Summary:
related to T15996

- docs are now buildable with b2 (in a very simplified way) like mysql docs
- change Async.MQTT5 -> Boost.MQTT5, async_mqtt5 namespace to boost::mqtt5 namespace

- once changes are approved, Ill update all tabs to 4 spaces in .qbk files

Reviewers: ivica

Reviewed By: ivica

Subscribers: iljazovic, miljen

Differential Revision: https://repo.mireo.local/D33373

example/timeout_with_awaitable_operators.cpp

@@ -25,8 +25,6 @@
 #include <boost/asio/ip/tcp.hpp>
 #include <boost/asio/steady_timer.hpp>
 
-#include <boost/mysql/any_connection.hpp>
-
 #include <chrono>
 #include <iostream>
 #include <string>