docs: simplify conf.py and fix compatibility with upcoming Sphinx 6.0

Change-Id: I970c5691cb4b1879854023a3747c2bf63189c128
diff --git a/docs/named_data_theme/named_data_header.html b/docs/named_data_theme/named_data_header.html
index c84397c..e6e81f1 100644
--- a/docs/named_data_theme/named_data_header.html
+++ b/docs/named_data_theme/named_data_header.html
@@ -26,7 +26,7 @@
     <div class="row">
          <div class="three columns">
               <div id="logo">
-                    <a href="http://named-data.net" title="A Future Internet Architecture"><img src="http://named-data.net/wp-content/uploads/cropped-20130722_Logo2.png" alt="" /></a>
+                    <a href="https://named-data.net" title="A Future Internet Architecture"><img src="https://named-data.net/wp-content/uploads/cropped-20130722_Logo2.png" alt="" /></a>
               </div><!--logo end-->
          </div>