Resolve "Fix CSS for front page" - Navigation

• Make menu responsive
• Allow to configure the navigation via navigation.yml again
• Use list elements instead of paragraphs
• Clean up
• Multiple optical improvements
• Make navbar scrollable on devices which are not big enough.

👉 Review here

@lisa.klaffki @englae39 @jandt-desy @erxleb87 @SpickerHZB Could you please comment or approve with respect to the navigation? It should now also be scrollable on smaller devices.

Closes #260 (closed)