DOC add link to sklearn_is_fitted example in check_is_fitted (scikit-learn#26926)

Co-authored-by: Arturo Amor <86408019+ArturoAmorQ@users.noreply.github.com>

Author: adrinjalali

sklearn/utils/validation.py

@@ -1410,8 +1410,10 @@ def check_is_fitted(estimator, attributes=None, *, msg=None, all_or_any=all):
     raises a NotFittedError with the given message.
 
     If an estimator does not set any attributes with a trailing underscore, it
-    can define a ``__sklearn_is_fitted__`` method returning a boolean to specify if the
-    estimator is fitted or not.
+    can define a ``__sklearn_is_fitted__`` method returning a boolean to
+    specify if the estimator is fitted or not. See
+    :ref:`sphx_glr_auto_examples_developing_estimators_sklearn_is_fitted.py`
+    for an example on how to use the API.
 
     Parameters
     ----------