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