Add extended Symfony YAML features to the YAML Format page?

[javiereguiluz]

After merging #18058 I realized that in https://symfony.com/doc/current/reference/formats/yaml.html we only show the YAML spec things that we support and mention the things we don't.

Should we also mention the Symfony-specific YAML features such as using PHP constants in YAML config files via the !php/const ... notation?

[MrYamous]

IMO this kind of feature needs to be more exposed as it can be very useful for a lot a developers in SF community and I think reference page for YAML is a good place to add this and others symfony-specific YAML features.

After more search, it's also mentioned in Yaml Component documentation but components documentation can be difficult to reach and this may not be the first place you think to look when you want to define your configuration and as Configuration parameters example but without more details about how it works.

[alexandre-daubois]

I gave it a try!