License headers in Markdown files visible

In Markdown-formatted files having a copyright/license header, the text is visible in the rendered output as the syntax is only suitable for RST.

We could either

• transform all MD files to RST
• or use this syntax:

<!-- comment
   SPDX-FileCopyrightText: 2015-2021 Sebastian Wagner
   SPDX-License-Identifier: AGPL-3.0-or-later
-->

(not urgent)