[Docs] Improve snarkVM repository documentation

[facundoolano]

• Add a README with high-level information to each component/crate of snarkVM, describing what the code does, how it relates to the bigger aleo ecosystem and any other usfeul context.
• Add module-level doc comments to all or at least the most relevant modules, explaining why the module is necessary and any context to complement the reading of its interface
• Add struct-level doc comments establishing what the struct represents and how it fits in the project