Move and revise code examples from hooks to events

For people reading this and not finding a good example, I just posted one here: https://www.drupal.org/project/search_api_solr/issues/3203375#comment-14... ✨ Introduce events and deprecate hooks Fixed

@mkalkbrenner, don't you think there could be some examples inside an examples folder? Just lik the hook_event_dispatcher module does? See: https://git.drupalcode.org/project/hook_event_dispatcher/-/tree/3.x/exam...

Feel free to provide a patch.

I've added a guide page explaining how to replace the deprecated hook_search_api_solr_query_alter with PreQueryEvent. Hopefully it helps some people.
https://www.drupal.org/docs/8/modules/search-api-solr/search-api-solr-ho... →

Thank you! I haven't tried it yet, but I am super appreciative of the effort!