[doc] Try to satisfy sphinx>=8

doc/conf.py

@@ -137,4 +137,4 @@ auto_intersphinx_packages = [
 auto_intersphinx_catalog = "catalog.json"
 
 # Add our private index (for extras and fixes)
-intersphinx_mapping = dict(extras=("", "extras.inv"))
+intersphinx_mapping = dict(extras=("https://mednet.rtfd.io", "extras.inv"))