Write proper introductory material about BB, with a proper attribution of BB5's history (currently only SA is credited, which is wrong)

<img width="1249" alt="Capture d’écran 2024-05-03 à 10 10 25" src="https://github.com/bbchallenge/bbchallenge-proofs/assets/7616128/e7ed107b-3b24-4bc1-8c40-8404a71cf7af">

https://discord.com/channels/960643023006490684/1151558585344593950/1235860350713200681
