Replaced the "Hello World" example to avoid confusion with AcmeDemoBundle
Reworded the beginning of one section
Replaced "AcmeWebsiteBundle" by "AcmeDemoBundle"
Added a note for users with an existing AcmeDemoBundle
feature #3888 Updated the example used to explain page creation (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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Updated the example used to explain page creation

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | #3576

To avoid confusions with the AcmeDemoBundle, this chapter now uses a "Random Number" example instead of the classic "Hello World".

Commits
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1350e6c Added a note for users with an existing AcmeDemoBundle
1c01b49 Replaced "AcmeWebsiteBundle" by "AcmeDemoBundle"
2345a0e Reworded the beginning of one section
575bc9a Replaced the "Hello World" example to avoid confusion with AcmeDemoBundle