Merge pull request matplotlib#15184 from dstansby/missing-ref-par

DOC: Mark missing_references extension as parallel read safe

Author: tacaswell

doc/sphinxext/missing_references.py

@@ -292,3 +292,5 @@ def setup(app):
     app.connect("builder-inited", prepare_missing_references_handler)
     app.connect("missing-reference", record_missing_reference_handler)
     app.connect("build-finished", save_missing_references_handler)
+
+    return {'parallel_read_safe': True}