[Routing] Add Attribute code examples for alias in #[Route] attribute

[welcoMattic]

Fixes #20600

This one adds code examples to alias route in #[Route] attribute. It also add example using DeprecatedAlias.

I've also added a note to precise new_route_name and original_route_name meaning, to avoid misunderstanding.

cc @damienfern

[welcoMattic]

@stof for review

I've fixed issues we talk about in private.

But, as we discussed, we need to make this Route Alias part of the documentation a bit clearer on lower branches. This PR documents the new way to create route aliases using Attributes (which differs from the way to do in YAML, XML and PHP, if I've understood correctly).