A11y audit fixes

This MR fixes some issues raised in the accessibility audit (#26). In addition, it fixes other issues found using Wave.

• Wrap logo and main navigation in a ˋheaderˋ element
• Wrap the main content in a ˋmainˋ element
• Add invisible "skip link" to directly jump to content
• sections need a heading, check if ˋarticleˋ might be a better fit
• All pages need to have a ˋh1ˋ element
• Use a single ˋh2ˋ for subtitles on the main website
• Increase contrast ratio in the Footer and Buttons (There was a bug, I'll ask if that is fixing the issue)
• Consider a dark mode (this is probably breaking the scope here) #27
• Remove alt text for decorative icons
• Simplify alt text for descriptive images