Reorder page from simple to advanced (rebased)

As https://github.com/symfony/symfony-docs/pull/3910 got meshed up here is a cherrypicked version.

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As a followup on symfony/symfony#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.

Fixes #3910