[Routing] Adding full example for a Service in Route Condition

[ThomasLandauer]

Reasons:

• There needs to be an example on how to pass any information to this callback service. I merely guessed the route syntax - it's not even explained at https://symfony.com/blog/new-in-symfony-6-1-services-in-route-conditions - is there a better way to do it?
• How to call a method from this service wasn't explained (the .check() part)
• This method needs to return bool?

I don't think it's a good idea to add those code blocks inside this indented "list" - will it even work? So probably it's better to create a dedicated paragraph (or even heading) for Services in Route Conditions. But you need to tell me how (i.e. what structure you have in mind).

[javiereguiluz]

Thanks Thomas!

While merging, I changed the order of the two PHP code samples to show the service first and the route condition second. See 8c3cb80