Update docs/configuring-playbook-synapse-auto-accept-invite.md: add the copyright header

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

docs/configuring-playbook-synapse-auto-accept-invite.md

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+<!--
+SPDX-FileCopyrightText: 2024 MDAD project contributors
+SPDX-FileCopyrightText: 2024 Slavi Pantaleev
+SPDX-FileCopyrightText: 2024 Suguru Hirahara
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
 # Setting up Synapse Auto Invite Accept (optional)
 
 The playbook can install and configure [synapse-auto-invite-accept](https://github.com/matrix-org/synapse-auto-accept-invite) for you.