Bring handbook design more in line with our Brand Guidelines

[felixfbecker]

This makes our handbook styles follow the Brand Guidelines more closely. Concretely:

• Use our font (Source Sans/Code)
• Use our color palette
• Update all logos
• Tweak font sizes and line heights (I tried to stick to the guidelines, but had to make some exceptions for some headings because content didn't fit well)

Other changes:

• Remove dead CSS I noticed (if you see deleted CSS in the diff, it was never used, there is CSS later in the file overriding it)
• Use SVG favicon
• Make logo a bit larger
• Make # when hovering a heading scale to font size of heading
• There are also some autoformatting/consistency changes (I would really love to run Prettier over the whole repo sometime soon and set some automatic GitHub action up)

This is a bit of a prerequisite for my PR to add the career levels because I want those to look good, but extracted this part into its own PR.

Before:

After:

[eseliger]

Nice 🌟