add header cryptpad 5.6.0 (#60)

Reviewed-on: #60
Reviewed-by: muppeth <muppeth@no-reply@disroot.org>

templates/etc/nginx/sites-available/cryptpad.j2

@@ -1,3 +1,7 @@
+# SPDX-FileCopyrightText: 2023 XWiki CryptPad Team <contact@cryptpad.org> and contributors
+#
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
 {% extends "core.j2" %}
 
 {% block app_specific %}