Change wording and include reference to previous versions of Plone documentation

Author: stevepiercy

docs/_templates/404.html

@@ -7,7 +7,14 @@
 {% block body %}
 <div>
     <h1>Page not found</h1>
-    <p>Unfortunately we couldn't find the content you were looking for. Use this site's <a href="/search.html">search</a> to find what you seek.</p>
+    <p>We could not find the page you requested in Plone 6 documentation.</p>
+    <p>You can <a href="/search.html">search</a> Plone 6 documentation.</p>
+    <p>Previous versions of Plone documentation:</p>
+    <ul>
+        <li><a href="https://5.docs.plone.org/">Plone 5</a></li>
+        <li><a href="https://4.docs.plone.org/">Plone 4</a></li>
+        <li><a href="https://3.docs.plone.org/">Plone 3</a></li>
+    </ul>
 </div>
 
 <style>