Different start method in Get Started guide and starter

Fixes #162

The snippet used a different start method declaration than the start
method generated by the starter.
Users could get confused by copy/pasting only the body of the method and
missing to complete/or fail the start promise.

Besides, the sample didn't have imports so the user had to figure
them out without any indication.

Also, updated the screenshot (latest version of the starter).

pages/get-started.mdx

@@ -65,9 +65,18 @@ Change the code as follows:
 <CodeExample title="Java">
 
 ```java
+package com.example.starter;
+
+import io.vertx.core.AbstractVerticle;
+import io.vertx.core.MultiMap;
+import io.vertx.core.Promise;
+import io.vertx.core.json.JsonObject;
+import io.vertx.ext.web.Router;
+
 public class MainVerticle extends AbstractVerticle {
+
   @Override
-  public void start() throws Exception {
+  public void start(Promise<Void> startPromise) throws Exception {
     // Create a Router
     Router router = Router.router(vertx);
 
@@ -93,12 +102,16 @@ public class MainVerticle extends AbstractVerticle {
       .requestHandler(router)
       // Start listening
       .listen(8888)
-      // Print the port
-      .onSuccess(server ->
-        System.out.println(
-          "HTTP server started on port " + server.actualPort()
-        )
-      );
+      // Print the port on success
+      .onSuccess(server -> {
+        System.out.println("HTTP server started on port " + server.actualPort());
+        startPromise.complete();
+      })
+      // Print the problem on failure
+      .onFailure(throwable -> {
+        throwable.printStackTrace();
+        startPromise.fail(throwable);
+      });
   }
 }
 ```