Consider moving to MkDocs

Just an idea. Pros:

• Simpler stack, simpler config.
• Markdown has won. People don't know reST, even myself, after years of using it, still make the most basic mistakes and need to lookup the most basic stuff.
• Also supported by RTD.
• Has the Material theme, which is modern, under active development, flexible, has dark theme, etc.
• Is used at other projects, such as python.cz or junior.guru, so know-how would be shared (contributions, maintenance).

Cons:

• We extend Sphinx and would need to rewrite those things.
• We use reST extensively. I'm not sure we can port it 1:1 (but is it necessary?)

How:

• This can happen gradually. Our current setup supports Markdown, so texts can be converted first and we would see.
• I already moved from builtin Sphinx link check to Lychee, as it works better.

Überprüfe die aktuelle Sphinx-Konfiguration sowie die vorhandene Markdown- und reST-Dokumentation und vergleiche anschließend die benutzerdefinierten Erweiterungen mit MkDocs und dessen Material-Theme. Prüfe die vorhandene Lychee-Konfiguration zur Linkprüfung und die Machbarkeit einer schrittweisen Markdown-Konvertierung. Im Issue werden weder Dateien noch Akzeptanzkriterien genannt, daher müssen der vereinbarte Migrationsumfang und die Definition of Done noch festgelegt werden.