UI Redesign in line with new Website update

[fhennig]

reference docs pr: stackabletech/documentation-ui#73

new website draft: https://stackable.berlin-byt.es/

• change header
• change footer
• change fonts

stretch goals:

• get rid of halfmoon dependency
• improve accessibility
• maintenance, dependency bumps

[fhennig]

I want to spend some more time on getting rid of the halfmoon JS script, and making the menu more in line with the other sites.

For that I want to also build a popup menu that is triggered with a click (for accessibility and mobile, and so I don't have to build an alternative mobile version right away) but build it with plain JS and make it look more like the menu on the main site.

[fhennig]

Done. I've made the popup menu use simple JS, and had to replace the "Copied" tooltip with something custom as well.