Improve docs structure and design

This MR changes the sphinx theme to Sphinx Book Theme, in line with the redesign of the Utopia docs in utopia!256 (merged).

This also adds a logo for dantro, picking up the design of the Utopia logo 🎉

Can this MR be accepted?

• Implementation ready
• Documentation extended or updated
• Added logo to README

• Ready for merging
  • New design looks good
  • !259 (merged) merged (fixes a failing test which currently blocks this MR)
  • Pipeline passes without warnings
  • History cleaned up
  • Changelog entry added
  • Version number bumped
  • Approved

Related issues

Closes #283 (closed)