[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 wouldmchange the second sentence with "Tou can also run the server ..."

The "Since ..." part is already covered by the versionadded directive