Add copyright header to docs/obtaining-access-tokens.md

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

docs/obtaining-access-tokens.md

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+SPDX-FileCopyrightText: 2022 Dennis Ciba
+SPDX-FileCopyrightText: 2022 Slavi Pantaleev
+SPDX-FileCopyrightText: 2025 MDAD project contributors
+SPDX-FileCopyrightText: 2024 - 2025 Suguru Hirahara
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+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
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 # Obtaining an Access Token
 
 When setting up some optional features like bots and bridges you will need to provide an access token for some user. This document provides documentation on how to obtain such an access token.