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

Refer f8b779f8ed as well

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

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

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+<!--
+SPDX-FileCopyrightText: 2022 - 2024 MDAD project contributors
+SPDX-FileCopyrightText: 2022 - 2025 Slavi Pantaleev
+SPDX-FileCopyrightText: 2023 Nikita Chernyi
+SPDX-FileCopyrightText: 2024 - 2025 Suguru Hirahara
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+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
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 # Setting up a Generic Mautrix Bridge (optional)
 
 The playbook can install and configure various [mautrix](https://github.com/mautrix) bridges (twitter, discord, signal, googlechat, etc.), as well as many other (non-mautrix) bridges. This is a common guide for configuring mautrix bridges.