update the documentation about bin/console server:...

[SimonHeimberg]

Since symfony 3.3, for starting the internal webserver, symfony/web-server-bundle must be installed and registered. Please update the documentation.

Documentation to update:

• https://symfony.com/doc/current/setup/built_in_web_server.html
• https://symfony.com/doc/current/setup.html#running-the-symfony-application

related bug symfony/symfony#21373 has been fixed by adapting documentation symfony/symfony#22094

[SimonHeimberg]

At my (upgraded) installation, symfony/web-server-bundle was installed (with framework bundle) but not registered.