Add copyright header to docs/configuring-playbook-base-domain-serving.md

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

docs/configuring-playbook-base-domain-serving.md

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+SPDX-FileCopyrightText: 2019 - 2024 Slavi Pantaleev
+SPDX-FileCopyrightText: 2024 Suguru Hirahara
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+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
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 # Serving the base domain (optional)
 
 By default, this playbook sets up services on your Matrix server (`matrix.example.com`), but has it configured so that it presents itself as the base domain (`example.com`). To have this server officially be responsible for Matrix services for the base domain (`example.com`), you need to set up server delegation / redirection.