Additional references on stateless SPV

[mhluongo]

Our treatment of stateless SPV in the docs is pretty sparse.

• [ ] Add references to the actual SPV proof mechanics
• [ ] Consider adding a simplified code snippet to show proof verification
• [ ] Cite Summa's existing work
• [ ] Point to BTCRelay on-chain and give more historical context around its failure

[gakonst]

• [ ] Consider citing the potential security pitfalls of Stateless SPV
• [ ] Consider describing Summa's improvements over BTCRelay