Update the Getting Started tutorial to follow the Symfony Best Practices

[javiereguiluz]

There are some pending issues to be merged in the symfony/symfony-standard repo related to the new Symfony best practices. Basically we'll provide an AppBundle already created and a services.yml configuration file.

The tutorial aimed at newcomers should use the existing AppBundle and the already created config files. This issue is a reminder to update that tutorial. If the doc managers agree, I'd like to take care of this issue myself.

[xabbuh]

@javierguiluz If you want to do all the work, I won't mind. :)

[javiereguiluz]

Closing this issue because the related PR is merged and this is now fixed.