Use standard bundle installation template

*The Symfony Documenators started an initiative to make all bundle installation documentation consistent and  easy to understand for beginners. That'll improve the DX for starters using 3rd party bundles. The template can be found here: http://symfony.com/doc/current/cookbook/bundles/best_practices.html#installation-instructions*

I would like to ask you to use this template in your documentation.

If you have any comments on the template, please open an issue in the `symfony/symfony-docs` repository. Feedback will brings us to the ultimate template.
already fixed in another PR