[JsonEncoder] Add JsonEncodable attribute

[chalasr]

Q	A
Feature PR	symfony/symfony#59401
PR author(s)	@mtarld
Merged in	7.3

We created this issue to not forget to document this new feature. We would really appreciate if you can help us with this task. If you are not sure how to do it, please ask us and we will help you.

To fix this issue, please create a PR against the 7.3 branch in the symfony-docs repository.

Thank you! 😃