Added standard bundle installation instructions

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | -
| Fixed tickets | #4157

Note: if you think that this draft is awful, please say it so and we'll rewrite it from scratch.
Yeah, I think it's wrong. A sidebar shows something that is not the main content, but just a little "side information". Things that are good to know, etc. (the "How is this event used in the Symfony Framework" sidebars of the HttpKernel docs are a good example of this).

In this case, it is the most important information of the complete section.