Follow REUSE Guidelines for Licensing

[mhellmeier]

🐞 Bug Report

Describe the Bug

Currently, the structure and usage of license headers aren't fully harmonized in the whole project. Fore example, same Markdown files contain license headers while others don't.

Expected Behavior

A harmonized strategy towards all project components, including things like SPDX-FileCopyrightText.

Additional Context

In order to solve the problem, this project should follow the REUSE guidelines.
Further, the CONTRIBUTING guidelines need to be updated with additional information.