Add comments next to the code indicating where in the documentation they are used.

I’m currently reading this tutorial to learn how to use Jakarta EE. And, as I read through it, I’d like to help improve the code snippets where I think it’s necessary.

But I realise that if I change any code, it might leave some examples in an inconsistent state. That’s why I think I could add a comment next to the code indicating which tutorials they are used in. That way, anyone contributing to improving the examples will also know where to look to change what is shown in the tutorials.

For example, at [The rsvp example application](https://jakarta.ee/learn/docs/jakartaee-tutorial/current/websvcs/rest/rest.html#_the_rsvp_example_application) several snippets from the [rsvp](https://github.com/eclipse-ee4j/jakartaee-examples/tree/main/tutorial/jaxrs/rsvp) example are used.
So I could add a comment above the code used that says:
//used in https://github.com/jakartaee/jakartaee-tutorial/blob/main/src/main/antora/modules/websvcs/pages/rest/rest003.adoc
Or something along those lines. Please let me know if you think this is correct so I can start making those changes as I work through the tutorial.