Rework user-facing configuration (mostly website)

The configuration for the website needs some rework:

• Merging configurations via yaml dict is an antipattern and results in unexpected results. As example, the navbar cannot be removed entirely because of this merging. For some (like the legal padding amount), it should have a default, for others not.
• Mdbook theme and theme selector are replaced by our own implementation / theme. It would be nicer to add a theme, and not replace everything
• It would also be nice to just allow going with mdbook defaults. Any modifications on top need to be kept up to date with mdbook, which entails a maintenance burden. We would also just like being able to go with the defaults.
• It seems like some values (such as the root colors in the default_config.yml) are unused. The colors are not strings and hence commented out.