Adds 404 error page in default configs.

doc/debian/jitsi-meet/jitsi-meet.example

 
     root /usr/share/jitsi-meet;
     index index.html index.htm;
+    error_page 404 /404.html;
 
     location /config.js {
         alias /etc/jitsi/meet/jitsi-meet.example.com-config.js;