Explicitly ignore legacy file reference errors in sphinx -n mode

docs/extensions/nitpick_file_ignorer.py

@@ -0,0 +1,22 @@
+import logging
+
+from sphinx.application import Sphinx
+from sphinx.util import logging as sphinx_logging
+
+
+class NitpickFileIgnorer(logging.Filter):
+    
+    def __init__(self, app: Sphinx) -> None:
+        self.app = app
+        super().__init__()
+
+    def filter(self, record: sphinx_logging.SphinxLogRecord) -> bool:
+        if getattr(record, 'type', None) == 'ref':
+            return record.location.get('refdoc') not in self.app.config.nitpick_ignore_files
+        return True
+
+
+def setup(app: Sphinx):
+    app.add_config_value('nitpick_ignore_files', [], '')
+    f = NitpickFileIgnorer(app)
+    sphinx_logging.getLogger('sphinx.transforms.post_transforms').logger.addFilter(f)