Updating component usage to use composer require

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | n/a

Hi guys!

This shows using the new `composer require` command instead of manually updating `composer.json`. It shortens and simplifies things quite a bit :grinning: 

Thanks!
I know we had this before, but I'm not sure if we should really keep it. The `symfony/symfony` will not only install all the components, but will also, for example, install the core bundles. It's useful if you want to run tests against a specific Symfony version. Though I don't think we should teach people to use it as a general dependency.