Added Vagrant configuration cookbook

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | n/a

A new cookbook article was added with an example Vagrant configuration that can be used with the Symfony Standard Edition.
While it is true that Vagrant is compatible with a number of providers (Virtualbox, VMWare, Docker, Hyper-V, etc.), one might need to have prior knowledge about how to set them up or will have to purchase them.  Mentioning that just seems unnecessary and might confuse someone (who might be new to this) about which provider to install and, thus, not allow them to complete the goal of trying out Symfony within the new Vagrant virtual machine.

From https://docs.vagrantup.com/v2/providers/default.html

> By default, VirtualBox is the default provider for Vagrant. VirtualBox is still the most accessible platform to use Vagrant: it is free, cross-platform, and has been supported by Vagrant for years. With VirtualBox as the default provider, it provides the lowest friction for new users to get started with Vagrant.