set up as quarto project

Quarto features projects as a way to build a bunch of quarto documents together. I think this is advantageous for our project, because it:

• allows to share metadata and styles etc... between quarto documents
• enables pre- and post- processing
• can build a little website around
• simplifies handling of multiple documents

I tried to include !5 (closed), the summary will end up as a separate web page. This will (just slightly) conflict with !4 (closed), because I've moved the lectures into a separate subfolder (lectures) which will simplify common metadata and styles in turn. It should work with !3 (merged), although I guess in medium-term building the presentations should become even simpler as a bunch of things which we would have to loop over manually is taken care by quarto.