Chore/general overhaul

[tumidi]

• Adapt nav tree to match new structure from #5.
• Add a neutral color palette (can be decided on later), QuestionPy logo and enable dark mode (avoiding flash bang 🫠)
• Add contributions page skeleton.
• Add cross-references to Python standard lib and Pydantic (similiar to Intersphinx).
• Remove <p style="text-align:justify">...</p> in Markdown. If justified text is really wanted, it should be implemented in a consistent manner in CSS.
• Make about first page tab and move all project info there, also the index page now redirects to about/index.md.
• Other minor changes...

Merge #5 first