NFT API methods

[mDuo13]

The XLS-20 implementation (currently previewed on NFT-Devnet) has 3 new API methods:

• account_nfts
• nft_sell_offers
• nft_buy_offers

Add API reference docs for all three of these plus any other methods that are part of the scope of the XLS-20 feature.

Also add example requests to the WebSocket API tool examples

[mDuo13]

This is largely done, but the NFT concept page's list of the related methods should link the new pages.