Update docs/configuring-playbook-ntfy.md: add a copyright header

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-ntfy.md

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+<!--
+SPDX-FileCopyrightText: 2022 - 2024 Slavi Pantaleev
+SPDX-FileCopyrightText: 2022 Julian Foad
+SPDX-FileCopyrightText: 2022 MDAD project contributors
+SPDX-FileCopyrightText: 2023 Felix Stupp
+SPDX-FileCopyrightText: 2024 - 2025 Suguru Hirahara
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
 # Setting up the ntfy push notifications server (optional)
 
 The playbook can install and configure the [ntfy](https://ntfy.sh/) push notifications server for you.