Add copyright header to docs/configuring-captcha.md

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

docs/configuring-captcha.md

@@ -1,3 +1,12 @@
+<!--
+SPDX-FileCopyrightText: 2020 Justin Croonenberghs
+SPDX-FileCopyrightText: 2020 - 2024 Slavi Pantaleev
+SPDX-FileCopyrightText: 2022 MDAD project contributors
+SPDX-FileCopyrightText: 2024 Suguru Hirahara
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
 (Adapted from the [upstream project](https://github.com/element-hq/synapse/blob/develop/docs/CAPTCHA_SETUP.md))
 
 # Overview