Re-design homepage ai-validation website

[anneschuth]

Implementation Notes

• ideally the top navigation (and not the logo) is sticky
• blocks on the frontpage are snappy
• page in english
• other pages stay as they are, but we check that at least color scheme is applied there as well
• we stick with mkdocs-material for now (but if material is an obstacle, we may want to opt for vanilla mkdocs)
• search box is extandable
• the image on the frontpage should not have padding on the right
• the page should be responsive
• we use overrides (look at mkdocs-material for orignals that should be copied as a starting point)
• if it is hard to change search/github in the header, we keep it as is