Edit nav, add imageAlt, add placeholder video

Webauftritt/eportfolio/impressions.html

@@ -10,6 +10,10 @@
 
 <h1 class="flex justify-center">Beispiele</h1> <!--todo refine the positioning of text on page-->
 
+<iframe width="560" height="315" src="https://www.youtube.com/embed/dQw4w9WgXcQ" title="YouTube video player"
+        allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture"
+        allowfullscreen class="self-center"></iframe>
+
 <script type="module" src="/main.js"></script>
 </body>
 </html>