refactor: Log a debug message for unresolved optional references

This can help identify external references that could be resolved by loading additional inventories.

src/mkdocs_autorefs/references.py

@@ -391,6 +391,7 @@ def inner(match: Match) -> str:
             url = _find_url(identifiers, url_mapper)
         except KeyError:
             if optional:
+                log.debug("Unresolved optional cross-reference: %s", identifier)
                 if hover:
                     return f'<span title="{identifier}">{title}</span>'
                 return title