[DOC] doc: improve navigation to main doc page

also, fixes a small error

doc/event_bus.md

@@ -5,7 +5,7 @@ parts of the code. Owl has a very simple bus class, which manages subscriptions,
 triggering events, and callbacks.
 
 ```js
-const bus = new owl.EventBus();
+const bus = new owl.core.EventBus();
 
 bus.on("some-event", null, function(...args) {
   console.log(...args);