Bootstrapped article about removing bundles (especially the AcmeDemoBundle)

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | -

This cookbook article explains how to savely remove a bundle from your application and it takes the AcmeDemoBundle as an example.

This is discussed a couple times (symfony/symfony#1309 and symfony/symfony-standard#412). The conclusions of those discussions where to create "**clear instructions on how to remove the Acme demo code by hand.**" and not do it automatically, it will get to complicated.

I will squash the commits together if it get merged, but first I want to hear what I should improve :)