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.
Fixed feedback from @xabbuh
Fixed feedback from @xabbuh: Templates
Added feedback from @weaverryan with a little move around.
Fixed @weaverryan: backticks and rephase method overrides.
feature #3928 Reorder page from simple to advanced (rebased) (clemens-tolboom)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #3928).

Discussion
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Reorder page from simple to advanced (rebased)

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

===
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

Commits
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13f5ec6 Fixed @weaverryan: backticks and rephase method overrides.
3ad1c6c Added feedback from @weaverryan with a little move around.
10ed835 Fixed feedback from @xabbuh: Templates
67e89af Fixed feedback from @xabbuh
334492a Reorder page from simple to advanced