Update the documentation to take into account the new AppBundle

[javiereguiluz]

Current documentation uses the classic AcmeDemoBundle namespace in lots of examples and code samples. We should consider moving everything to AppBundle, which will be always available once the pending Symfony Standard PR is merged.

[javiereguiluz]

Closing this issue because it was a reminder and this task has already been tackled by other issues related to the Getting Started tutorial and the book.