Reorder page from simple to advanced

As a followup on https://github.com/symfony/symfony/pull/10980 I want to propose to reorder the page as the obvious way should come first. I took me way to long to find the correct way :p

I added two headers

- The default ExceptionController
- Override error templates

in order to introduce how override works and moved the controller override to the bottom as that is more advanced.
How about this re-wording (or something like this). I want to give the user as much of a step-by-step guide as possible:

>
If you need a little more flexibility beyond just overriding the template (e.g. you need to pass some additional variables into your template), then you can override the controller that renders the error page.

>
To do this, create a new controller class and make it extend Symfony's default ``Symfony\Bundle\TwigBundle\Controller\ExceptionController`` class. There are several methods you can override to customize different parts of how the error page is rendered.

>
To make Symfony use your exception controller instead of the default, set the :ref:`twig.exception_controller <config-twig-exception-controller>` option in ``app/config/config.yml``.