Add copyright header to docs/importing-synapse-media-store.md

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

docs/importing-synapse-media-store.md

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+<!--
+SPDX-FileCopyrightText: 2019 - 2020 Slavi Pantaleev
+SPDX-FileCopyrightText: 2024 - 2025 Suguru Hirahara
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+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
 # Importing `media_store` data files from an existing Synapse installation (optional)
 
 You can manually import your `media_store` files from a previous installation of Synapse.