Add copyright header to docs/configuring-playbook-federation.md

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

docs/configuring-playbook-federation.md

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+<!--
+SPDX-FileCopyrightText: 2018 - 2024 Slavi Pantaleev
+SPDX-FileCopyrightText: 2022 - 2024 MDAD project contributors
+SPDX-FileCopyrightText: 2024 - 2025 Suguru Hirahara
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
 # Controlling Matrix federation (optional)
 
 By default, your server federates with the whole Matrix network. That is, people on your server can communicate with people on any other Matrix server.