Allow to rebuild docs with Warning this is an old version.

docs/Makefile

@@ -137,6 +137,8 b' gh-pages: clean html'
 	# if VERSION is unspecified, it will be dev
 	# For releases, VERSION should be just the major version,
 	# e.g. VERSION=2 make gh-pages
+	# use ARCHIVE=1 VERSION=3 gh-pages to build the
+	# docs with a warning that this is not stable docs anymore.
 	python gh-pages.py $(VERSION)
 
 texinfo:

docs/source/conf.py

@@ -77,6 +77,15 b" if iprelease['_version_extra'] == 'dev':"
 
     """
 
+elif os.environ.get('ARCHIVE', None):
+    rst_prolog = """
+    .. warning::
+
+        This documentation is for an old version of IPython.
+        You can find docs for newer versions `here <http://ipython.readthedocs.org/en/stable/>`_.
+        
+    """
+
 # The master toctree document.
 master_doc = 'index'