Add Subgraphs usage to NFT Marketplace tutorial

[Ginowine]

I think that since Subgraphs by Goldsky is one of the projects leveraging IOTA and is a good use case for building an NFT marketplace, it might be a good idea to use them for the NFT marketplace tutorial. .

We can use Subgraphs to read and sync data from the blockchain to the NFT marketplace.

This is related to issue #1651