Add hook_delete

[ekes]

Background:

• The module maintains a list of circular references Fix or document circular references #6 overview <-> pages.
• The overview list of referenced pages is updated when a page is updated (added/moved/removed from overviews) Fix/navigation unpublish move #27
• The code was also not checking if the references still existed and were still published Unpublished pages showing on GuidePrevNextBlock #44 Error: Call to a member function toUrl() on null in Drupal\localgov_guides\Plugin\Block\GuidesPrevNextBlock->build()  #31
• Using reference guide pages was incidentally fixed with Fix prev / next unpublish #44 #47 as explained in Deleted guide pages #49

Additional context:

It is normal in Drupal to not update the entities that are referencing an item that is deleted. The expectation is on any code using the list to check that they still exist. I remember it's tripped up Search API several times. There's a core issue to alter this behaviour https://www.drupal.org/project/drupal/issues/2723323

Task:

So despite the tests now passing as we are checking referenced entities exist before using them; but because we are handling updates, and core would like to handle deletions in the future. For consistency, we should add a hook_delete that corrects the overview/page entity references on deletion. This could offer the opportunity to remove all pages if the overview is deleted for example?