subnet NFT contract example

[elenananana]

@obycode suggested we might be able to use the nft use case demo (or parts of it) for a contract example in platform. Ideally, there is a simplified contract example (not necessarily the entire end to end experience) since subnets is an advanced use case.

This could be good for May - when subnets is on mainnet.

As far as the code is commented with sufficient details about the functionality, it could help devs understand it without having to refer to another MD file.

Limitation to consider: contracts in the example cannot be deployed without Subnets support in the chain. perhaps it's better to wait on it until we know the Subnets lands in Mainnet.