Update endpoint documentation to have @BrowserCallable as the primary way

[Legioth]

The documentation in https://vaadin.com/docs/latest/hilla/guides/endpoints describes @Endpoint as the primary annotation and mentions @BrowserCallable as a newly introduced alternative alias. We should invert the descriptions to instead describe @BrowserCallable as the main option and then also mention that the old @Endpoint name remains as a backwards compatibility option.