DOC: Enable intersphinx mapping to sklearn

Sphinks reads the public methods of all classes, and if one inherits from
sklearn, it reads that method's docstring.  If that method contains a relative
link in the rst, sphinx needs to know where to link it.

docs/conf.py

@@ -61,7 +61,8 @@
 html_sourcelink_suffix = ""
 
 intersphinx_mapping = {
-    "derivative": ("https://derivative.readthedocs.io/en/latest/", None)
+    "derivative": ("https://derivative.readthedocs.io/en/latest/", None),
+    "sklearn": ("https://scikit-learn.org/stable/", None),
 }
 
 # -- Extensions to the  Napoleon GoogleDocstring class ---------------------