[EventDispatcher] Document the #AsEventListener attribute

[94noni]

Document the AsEventListener attribute

Ref symfony/symfony#47015 (did not found any doc thus opened an issue at first)
For the code example, I've taken inspiration on the related PR:

• symfony/symfony@2ab3caf#diff-57b112e725705bbbf01380b0d11c9a8fe16ea69d85d1d3e618eceeb8b58094ce
• symfony/symfony@2ab3caf#diff-73a6a14d3288b358a21c0f47ecc1ec78c05e933b66e9a647e5013b2cab002b89

[javiereguiluz]

Thanks Antonie! Please note that while merging we moved these contents to the main event_dispatcher.rst article instead of keeping it in the article related to components. Cheers!