feat: rebuild the landing page

• implement the updated design
• use material theming helpers
• remove styled-components

Hi 👋🏽

Fixes lefat-stories#12 (closed)

Relates to lefat-stories#25

I rebuilt the Landing Page from scratch because I wasn´t that satisfied with the initial implementation from last weekend. Some things that are still lacking:

• Better mobile look and feel (I will create a separate story for that)
• Some more thought about the texts on the page (some of them don´t make that much sense in their respective context)
• Add some stuff in the footer
• Localization