Add merkle tree/merkle root explainer

The `commitBatches` function on the [ValidatorTimeLock](https://github.com/matter-labs/era-contracts//blob/4aa7006153ad571643342dff22c16eaf4a70fdc1/l1-contracts/contracts/zksync/ValidatorTimelock.sol) contract uses a merkle root hash of the state changes from the L2. So when explaining what this does in the boojum book, we should have a reference to explain what a merkle tree is/does. 