configure intersphinx crossref to importlib_resources docs

Signed-off-by: Oleg Höfling <oleg.hoefling@gmail.com>

importlib_metadata/docs/conf.py

@@ -166,6 +166,7 @@
 # Example configuration for intersphinx: refer to the Python standard library.
 intersphinx_mapping = {
     'python': ('https://docs.python.org/3', None),
+    'importlib_resources': ('https://importlib-resources.readthedocs.io/en/latest/', None),
     }