Adding overall document title

Doc/lectures.xml

@@ -9,6 +9,8 @@
       xmlns:html="http://www.w3.org/1999/xhtml"
       xmlns:db="http://docbook.org/ns/docbook">
   <info>
+    <title>Lecture notes</title>
+
     <author>
       <personname
       xlink:href="https://www.hdm-stuttgart.de/mi/kontakt/dozenten#goik"><firstname>Martin</firstname>