Put sunset in documentation

Author: pbugnion

docs/source/index.rst

@@ -3,8 +3,14 @@
    You can adapt this file completely to your liking, but it should at least
    contain the root `toctree` directive.
 
-IPywidgets server
-=================
+[Unmaintained] IPywidgets server
+================================
+
+IPywidgets server is no longer maintained. To serve widgets and rich
+outputs, we recommend using `voila
+<https://github.com/QuantStack/voila>`_ instead. See `this issue
+<https://github.com/pbugnion/ipywidgets_server/issues/48>`_ for a
+discussion.
 
 `IPywidgets server` lets you serve Jupyter widgets outside of a Jupyter
 Notebook. Any Python callback defined on your widgets will work as in the