[WCM] [Cookbook]Web server] description for running PHP's built-in web server in the background

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11311)
| Applies to    | 2.6+
| Fixed tickets | 

The work done to describe the new server helper commands is based on the cookbook article I proposed in #4000. So, that should be merged first. After that, this one can then be rebased on that changes.
I think that's beyond the scope of the cookbook article. If a user is interested in more details they can enter the PHP documentation that is linked at the beginning of this document. Usually, you wouldn't have to bother with the format since Symfony already provides router scripts for most use cases.