docs: add "emitting and listening to events" pages

Imported from the javascript documentation:

- https://socket.io/docs/v3/emitting-events/
- https://socket.io/docs/v3/listening-to-events/

src/site/site.xml

@@ -26,6 +26,8 @@
         <menu name="Overview">
             <item name="Installation" href="./installation.html"/>
             <item name="Usage" href="./usage.html"/>
+            <item name="Emitting events" href="./emitting_events.html"/>
+            <item name="Listening to events" href="./listening_to_events.html"/>
             <item name="Migrating from 1.x" href="./migrating_from_1_x.html"/>
         </menu>