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

[meaghanfitzgerald]

Suggested change

	See the documentation for the genesis JSON format [here](../genesis/README.md) and an example used for the local network genesis [here](https://github.com/ava-labs/avalanchego/blob/master/genesis/).
	See the documentation for the genesis JSON format [here](https://github.com/ava-labs/avalanchego/blob/master/genesis/README.md) and an example used for the local network genesis [here](https://github.com/ava-labs/avalanchego/blob/master/genesis/).

Because config.md is ported directly to the docs, relative links should be avoided so that a user viewing there doesn't get a dead link.

[meaghanfitzgerald]

Suggested change

	The Genesis package converts formatted JSON files into the genesis of the Primary Network. For the simplest example, see the [Local Genesis](./genesis_local.json) JSON file.
	The Genesis package converts formatted JSON files into the genesis of the Primary Network. For the simplest example, see the [Local Genesis](https://github.com/ava-labs/avalanchego/blob/master/genesis/genesis_local.json) JSON file.

This suggestion is only necessary if we also want to import this page into the docs. imo we should cc: @ashucoder9