Rebrand Simple NFT Challenge as Tokenization Challenge

[escottalexander]

We are going to change the simple NFT challenge name to "Tokenization" and update the image and README to reflect that.

The idea is that we will still give the same takeaways but tweak it away from "NFT" as a cultural phenomenon and lean into the concept of tokenizing anything (RWAs, Zebras, Buffalos) in the form of an ERC-721 (yes - an NFT). It is crucial that this challenge still helps students to understand the concept of ownership in the context of blockchain.

• Update README
• Update Image assets. @andrealbiac Should be informed about the image asset changes from Austin. 🙏
• Challenge metadata in SRE-v2
• Auto grader tests (if altered)