Update docs/alternative-architectures.md: add a copyright header

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

docs/alternative-architectures.md

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+SPDX-FileCopyrightText: 2020 - 2022 Slavi Pantaleev
+SPDX-FileCopyrightText: 2020 Horvath Gergely
+SPDX-FileCopyrightText: 2024 Suguru Hirahara
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+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
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 # Alternative architectures
 
 As stated in the [Prerequisites](prerequisites.md), currently only `amd64` (`x86_64`) is fully supported.