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

This is a preparation to copy the document to its own repository at the MASH project (https://github.com/mother-of-all-self-hosting/ansible-role-postgres-backup).

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

docs/configuring-playbook-postgres-backup.md

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+<!--
+SPDX-FileCopyrightText: 2021 foxcris
+SPDX-FileCopyrightText: 2021 - 2024 Slavi Pantaleev
+SPDX-FileCopyrightText: 2024 - 2025 Suguru Hirahara
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
 # Setting up postgres backup (optional)
 
 The playbook can install and configure [docker-postgres-backup-local](https://github.com/prodrigestivill/docker-postgres-backup-local) for you via the [ansible-role-postgres-backup](https://github.com/mother-of-all-self-hosting/ansible-role-postgres-backup) Ansible role.