[Documentation] Provide a howto documentation section on how to integrate piciquic with an external event manager

[fbussery]

Instead of using the internal "picoquic_packet_loop" function to run picoquic, we could need to use an external event manager with our own loop.

For instance, we could need to use instead an event loop based on Libevent or Boost.Asio.
A small howto page could make sense.
The proposition is to add a markdown file: doc/howto_use_external_event_manager.md
We can focus on a simple usage (unique path)

• explain how to push incoming udp to picoquic
• explain how application may push data to picoquic
• explain what does the event loop and when it is kicked
• how to handle callback

provide some pseudo code to illustrate the mechanism