[Doc] Improve DID Method Specifications

[abdulmth]

Description

Improve the method specs of the UTXO method.

To-do List

• [ ] Check if terms are up to date.
• [ ] Add diagrams. For example to explain how UTXO works.
• [ ] Explain how native tokens can be used with DID (debatable).
• [ ] Abstract away the stardust details in the CRUD operations.

[cycraig]

Is this issue completed?

[abdulmth]

@cycraig not really, none of the points has been done yet, especially abstracting the stardust details, which is also a debatable point.