Add impending deprecation notice and fix link for API (#6920)

* Add impending deprecation notice and fix link for API

* mention JEP 79

Author: RRosio

docs/source/extending/frontend_extensions.rst

@@ -1,3 +1,14 @@
+.. warning::
+
+   **DEPRECATED APIs** 
+    This documentation is for the Notebook 6 API which will soon be deprecated.
+    For up to date information on Notebook 7, please visit the latest version of the `Notebook documentation`_ or the `JEP for Notebook 7`_.
+    If you experience issues or are not yet able to migrate to Notebook 7, please visit the `NbClassic documentation`_ 
+
+.. _JEP for Notebook 7: https://jupyter.org/enhancement-proposals/79-notebook-v7/notebook-v7.html
+.. _Notebook documentation: https://jupyter-notebook.readthedocs.io/en/latest
+.. _Nbclassic documentation: https://nbclassic.readthedocs.io/en/latest/nbclassic.html
+
 Custom front-end extensions
 ===========================
 

docs/source/extending/handlers.rst

@@ -3,7 +3,7 @@ Custom request handlers
 
 The notebook webserver can be interacted with using a well `defined
 RESTful
-API <http://petstore.swagger.io/?url=https://raw.githubusercontent.com/jupyter/notebook/master/notebook/services/api/api.yaml>`__.
+API <http://petstore.swagger.io/?url=https://raw.githubusercontent.com/jupyter/notebook/6.5.x/notebook/services/api/api.yaml>`__.
 You can define custom RESTful API handlers in addition to the ones
 provided by the notebook. As described below, to define a custom handler
 you need to first write a notebook server extension. Then, in the