[PAGE ISSUE] Metadata schemas (add metaplex)

[murki]

Add section for metaplex token metadata schema https://docs.metaplex.com/token-metadata/specification primarily used for Solana NFTs.

Followed the instructions in https://nftschool.dev/reference/metadata-schemas/

This reference guide shows some of the most common and useful metadata formats used by NFTs today. If your favorite standard isn't included, please open an issue