Feature/document api respond parameters

[Jibbarth]

Hello,

As discussed in Slack, I create this PR to document a little bit the _api_respond parameters. I added it in the events page, but I don't know if it's enough or should I add more info.

I also add a big part on the DTO page, to explain how use it without an ApiResource declared.

My initial need was to retrieve a Settings custom in my application, but I didn't think it was a good example.
I thought also to propose the retrievement of the kernel.environment parameter, but I didn't find it pertinent.
Finally, browsing the symfony/demo repository, I found the Locales list, so I based my example on it.
Let me know if the example is pertinent or not.

Thanks all of you for your awesome works !

[dunglas]

Thanks @Jibbarth. I made some slight modifications to the text. Such PRs are very appreciated!

[dvc]

Typo:
public function __invoke(): LocalesDTO
should be
public function __invoke(): LocalesList