Parameterize a hardcoded version number

Author: mehaase

docs/_templates/header.html

@@ -6,7 +6,7 @@
       <a class="logo" href="{{pathto(master_doc)}}">
         <h1>{{theme_project_name}}
           <span class="version">
-            v2.2.7
+            {{version}}
           </span>
         </h1>
       </a>