Update docs/configuring-playbook-external-postgres.md: add a copyright header

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

docs/configuring-playbook-external-postgres.md

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+<!--
+SPDX-FileCopyrightText: 2018 - 2024 Slavi Pantaleev
+SPDX-FileCopyrightText: 2024 - 2025 Suguru Hirahara
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
 # Using an external PostgreSQL server (optional)
 
 By default, this playbook would set up a PostgreSQL database server on your machine, running in a Docker container. If that's okay, you can skip this document.