Resolve "Learning materials"

Review here

Closes #276 (closed) #236 (closed)

_includes/header.html

@@ -117,7 +117,7 @@ See also:
                             <strong>Subscribe to...</strong>
                         </summary>
                         <div>
-                            <p><a href="https://lists.desy.de/sympa/subscribe/hifis-announce">&nbsp;&nbsp;<i>Subscribe to News letter</i>&nbsp;<i class="fas fa-bell"></i></a></p>
+                            <p><a href="https://lists.desy.de/sympa/subscribe/hifis-announce">&nbsp;&nbsp;<i>Subscribe to Newsletter</i>&nbsp;<i class="fas fa-bell"></i></a></p>
                             <p><a href="{{ "feed/news.xml" | relative_url }}">&nbsp;&nbsp;<i>Subscribe to News RSS feed</i>&nbsp;<i class="fas fa-rss"></i></a></p>
                         </div>
                     </details>