Add note to built-in web server for using with a VM

[weaverryan]

See the conversation here: https://github.com/symfony/symfony-docs/pull/4000/files#r16325690

• Verify what IP address we should bind to inside of a VM (or determine that there is no one address you can bind to, but show how to look up the address)
• Add a sidebar to the built_in.rst file

Thanks!

[xabbuh]

The IP address to use is 0.0.0.0 (see this comment).