[Doctrine] Add documentation for Enum usage

[MrYamous]

Doctrine supports PHP native Enums for a while but i didn't find anything in the docs about it

I'm note sure about version to target. Enums are available from PHP 8.1 and SF 5.4 is 7.2+ then 6.4 looked good to me

[MrYamous]

I like this. Thanks Matthieu for contributing this.

However, we try to avoid creating vey small doc pages (because the require navigating from one small page to another and that frustrates most users). So, if it's possible, we should merge this as a new section in another article ... maybe inside the main doctrine.rst article? Thanks.

LGTM I will update this