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

Change-Id: I52415e37bcf237599aa5c4fa66605c34bc015626
diff --git a/docs/named_data_theme/named_data_header.html b/docs/named_data_theme/named_data_header.html
index c84397c..9179303 100644
--- a/docs/named_data_theme/named_data_header.html
+++ b/docs/named_data_theme/named_data_header.html
@@ -26,13 +26,13 @@
     <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>
 
          <!--top menu-->
          <div class="nine columns" id="menu_container" >
-           <h1><a href="http://named-data.net/doc/ndn-cxx/$projectnumber/">$projectname $projectnumber documentation</a></h1>
+           <h1><a href="https://named-data.net/doc/ndn-cxx/$projectnumber/">$projectname $projectnumber documentation</a></h1>
          </div>
     </div>
 </div><!--header container end-->