BREAK: add package name deprecation warning

src/sphinxcontrib/hep/pdgref/__init__.py

@@ -1,11 +1,22 @@
 """Link to PDG reviews and listings in Sphinx documentation."""
 
 from typing import Any, Dict
+from warnings import warn
 
 from sphinx.application import Sphinx
 
 from .role import URLPattern, pdgref
 
+warn(
+    (
+        "The sphinxcontrib.hep.pdgref package is deprecated. Please install the"
+        " [sphinx-hep-pdgref](https://pypi.org/project/sphinx-hep-pdgref) package and"
+        " import `sphinx_hep_pdg` instead."
+    ),
+    category=DeprecationWarning,
+    stacklevel=2,
+)
+
 
 def setup(app: Sphinx) -> Dict[str, Any]:
     app.add_role("pdg-listing", role=pdgref(pattern=URLPattern.LISTING))