Merge branch '345-custom-404-site' into 'preview'

Added 404 page

Closes #345

See merge request !631

doc.zih.tu-dresden.de/tud_theme/404.html

+{% extends "main.html" %}
+
+{% block content %}
+
+<center><img src="/assets/images/sad_smiley.png" alt="Sad smiley - page not found" width="200" height="200"/></center>
+
+<h3 style="text-align:center">The page you requested could not be found.</h3>
+<h3 style="text-align:center">Probably it has been moved, please use the search bar.</h3>
+
+<p style="text-align:center">If you have Javascript disabled, please enable it to see the search bar. </p>
+
+{% endblock %}