Resolve "Improve and restructure README"

This MR updates the documentation such that it is clearer what dantro does

Can this MR be accepted?

• Documentation extended or updated
  • Restructured README
  • Reworked dantro description in README and setup.py to better match what we write in the paper
  • Added description to main docs
• No code changes (except tested includes)

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added (not worth mentioning here, I think)
  • Version number bumped
  • Reviewed & approved by @herdeanu
  • Reviewed & approved by @jeremiastraub

Related issues

Closes #134 (closed)