hilla: add example usage of EndpointExposed

[cromoteca]

Include an example demonstrating how to use the @EndpointExposed annotation to control method exposure from superclasses in endpoint classes.

Closes #4202

[taefi]

LGTM!

[github-actions]

AI Language Review

For the updated Endpoints documentation, one primary issue requires attention:

1. New Section on Extending Other Classes:
   • The example provided for Extending Other Classes includes a code snippet demonstrating the use of @EndpointExposed. The explanation of how inherited methods are handled could benefit from a brief note to clarify the purpose of @EndpointExposed and why inherited methods from non-annotated superclasses aren't exposed by default. This clarification would improve comprehension for readers unfamiliar with the concept.