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.
Put 2 ticks (\`\`) around this class. But you're right not use the `:class:`, because I want the user to see the full class name :)