Merge pull request #3626 from gpaumier/patch-1

Fix broken link to add_node function in Sphinx doc

nikola/plugins/compile/rest/__init__.py

@@ -264,7 +264,7 @@ def add_node(node, visit_function=None, depart_function=None):
     """Register a Docutils node class.
 
     This function is completely optional. It is a same concept as
-    `Sphinx add_node function <http://sphinx-doc.org/extdev/appapi.html#sphinx.application.Sphinx.add_node>`_.
+    `Sphinx add_node function <https://www.sphinx-doc.org/en/master/extdev/appapi.html#sphinx.application.Sphinx.add_node>`_.
 
     For example::