Update docs/configuring-playbook-bridge-mautrix-hangouts.md: add a copyright header

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

docs/configuring-playbook-bridge-mautrix-hangouts.md

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+<!--
+SPDX-FileCopyrightText: 2019 Eduardo Beltrame
+SPDX-FileCopyrightText: 2019 - 2025 Slavi Pantaleev
+SPDX-FileCopyrightText: 2021 MDAD project contributors
+SPDX-FileCopyrightText: 2022 Dennis Ciba
+SPDX-FileCopyrightText: 2024 - 2025 Suguru Hirahara
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
 # Setting up Mautrix Hangouts bridging (optional, deprecated)
 
 🪦 The playbook used to be able to install and configure [mautrix-hangouts](https://github.com/mautrix/hangouts), but no longer includes this component, because Google Hangouts has been discontinued since the 1st of November 2022.