Update docs/just.md: add a copyright header

This file was created with e36115a5b9 based on fb60ba67f6 (announcement of adoption of just program).

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

docs/just.md

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+<!--
+SPDX-FileCopyrightText: 2023 - 2024 Slavi Pantaleev
+SPDX-FileCopyrightText: 2024 - 2025 Suguru Hirahara
+
+SPDX-License-Identifier: AGPL-3.0-or-later
+-->
+
 # Running `just` commands
 
 We have previously used [make](https://www.gnu.org/software/make/) for easily running some playbook commands (e.g. `make roles` which triggers [`ansible-galaxy`](https://docs.ansible.com/ansible/latest/cli/ansible-galaxy.html)). Our [`Makefile`](../Makefile) is still around, and you can still run these commands.