Added "How to create an event subscriber"

I've added the information about how to create an event subscriber to the "How to create an Event listener" cookbook recipe. I think that there isn't a recipe in the cookbook about that and it can be useful.
There's no need to use an external link here. You can just reference a document using the `doc` role like this:

```rst
[...] see :doc:`/components/event_dispatcher/introduction`.
```