[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 | 
Hmm, I think we should maybe keep the old section with some changes, because using the built-in PHP web server is not the only way to run things with the same user. But, I do think a link to the `built_in` document is a good idea.

Also, unrelated to this, I have 2 questions for you about the `server:*` commands themselves.

A) Do you think we could/should add a message to the `server:start` command?

>
To quit the server, run the server:stop command

B) Right now, if I already have a server running on, for example, port 8000, when I run `server:start`, it just gives me the normal "success" message: `Web server listening on http://127.0.0.1:8000`. While that's technically true, the reality is that the server did *not* start, because the port was already taken. Can we handle this better?

I wanted to ask you these here before opening an issue. If you agree and want to open an issue, that would be awesome :). Btw, I saw you moved the `pcntl` check - thanks for that.