docs: logo was removed since Sphinx 6 and replaced by logo_url

doc/_templates/layout.html

@@ -90,9 +90,8 @@
       <tr>
         <td valign="top" width="300">
           <h3><a href="{{ pathto('index') }}"><img
-          alt="C++ Boost" src="{{ pathto('_static/' + 'gil.png', 1) }}" border="0"></a></h3>
+          alt="C++ Boost" src="{{ pathto(logo_url, 1) }}" border="0"></a></h3>
         </td>
-
         <td >
           <h1 align="center"><a href="{{ pathto('index') }}"></a></h1>
         </td>