Fix architecture diagram font

Created by: felixfbecker

This diagram used to have a stylesheet that linked the font, but it was deleted accidentally a long time ago in ab4ddde8c96923df0e4e83b56578931ce1bc399f, leading to an ugly serif font fallback. This fixes it by adding a new stylesheet with the font. Also, adds dark mode 😎

I also moved the diagram to the top, as it got pretty lost at the bottom of the page and serves as a good overview I think.