Update docs/configuring-playbook-etherpad.md: add the copyright header

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

docs/configuring-playbook-etherpad.md

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+<!--
+SPDX-FileCopyrightText: 2021 Béla Becker
+SPDX-FileCopyrightText: 2021 - 2024 Slavi Pantaleev
+SPDX-FileCopyrightText: 2021 pushytoxin
+SPDX-FileCopyrightText: 2022 Jim Myhrberg
+SPDX-FileCopyrightText: 2022 Nikita Chernyi
+SPDX-FileCopyrightText: 2022 felixx9
+SPDX-FileCopyrightText: 2024 - 2025 Suguru Hirahara
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
 # Setting up Etherpad (optional)
 
 The playbook can install and configure [Etherpad](https://etherpad.org) for you.