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.
Agree! It's strange form the point of view of documentation source ... but I think it looks OK from the point of view of the documentation reader: [see a screenshot of the result](https://github.com/symfony/symfony-docs/pull/4163#discussion_r16607959)  If you still think that this is wrong, I'll change it.