Adds a doc page and example on multi-threading

doc/modules/ROOT/nav.adoc

@@ -6,6 +6,7 @@
 * xref:auth.adoc[]
 * xref:logging.adoc[]
 * xref:sentinel.adoc[]
+* xref:multi_threading.adoc[]
 * xref:benchmarks.adoc[]
 * xref:comparison.adoc[]
 * xref:examples.adoc[]

doc/modules/ROOT/pages/examples.adoc

@@ -18,6 +18,7 @@ The examples below show how to use the features discussed throughout this docume
 * {site-url}/example/cpp20_sentinel.cpp[cpp20_sentinel.cpp]: Shows how to use the library with a Sentinel deployment.
 * {site-url}/example/cpp20_subscriber.cpp[cpp20_subscriber.cpp]: Shows how to implement pubsub with reconnection re-subscription.
 * {site-url}/example/cpp20_echo_server.cpp[cpp20_echo_server.cpp]: A simple TCP echo server.
+* {site-url}/example/cpp20_echo_server_multithread.cpp[cpp20_echo_server_multithread.cpp]: A TCP echo server sharing a single connection between many sessions, running on a thread pool.
 * {site-url}/example/cpp20_chat_room.cpp[cpp20_chat_room.cpp]: A command line chat built on Redis pubsub.
 * {site-url}/example/cpp17_intro.cpp[cpp17_intro.cpp]: Uses callbacks and requires pass:[C++17].
 * {site-url}/example/cpp17_intro_sync.cpp[cpp17_intro_sync.cpp]: Runs `async_run` in a separate thread and performs synchronous calls to `async_exec`.