[SUGGESTION] Mutable NFTs -- Update with more context & extend with more alternatives

[dtbuchholz]

Overview

There was some work previously merged here about dynamic NFTs: #251. It's a great reference, but with the Mutable NFTs section, it makes sense to include some additional information and link to the dynamic NFTs section since this without it, this section is rather incomplete.

Requirements

There should exist more context in the "Background" section that points to the Metadata schemas section. There should also be callouts to an "Approach 3" that can further extend the information from Deeper & dynamic metadata with more detailed implementation information. Considering the existing section is all about dynamic NFTs toward the end, it makes sense to paint the whole picture here (e.g., centralized solutions are suboptimal, IPFS is great for files, smart contract mutability, etc.).