docs: rewrite readme (#8524)

## Description of changes

opening for discussion/feedback. this:

- makes the README cleaner
- quickly gets to bullet points for "Why", then links out to docs for
further reading
- quickly gets to code example
- other edits

we should also IMO update the GitHub description to "the portable Python
dataframe library" -- the "flexibility of Python with the scale and
performance of modern SQL" is in the REAMDE now

feedback welcome

## Issues closed

closes #8436 

cc: @chiphuyen @zhenzhongxu @ianmcook for feedback (in addition to the
Ibis team who will see this PR anyway)