move genesis docs to genesis package and add note on initial staker handling

[aaronbuchwald]

This PR moves the documentation for genesis JSON into the genesis/ package and adds an explanation of how funds for initialStakers are handled.

Specifically, if an address included in the genesis is marked as an initialStaker then the UTXOs described by its unlockSchedule are included in the the initial staker transaction instead of creating a UTXO in the genesis state of the P-Chain.

For reference, the flow is:

1. Initial stakers are added to a set here
2. When creating initial P-Chain UTXOs, we skip over any address included in initialStakers ref
3. We create the initial validators with their locked stakes here