src: document the code structure and flow

[xmonader]

• types, typesDevnet, typesLocal usage
• models and mappings?
  • seems the models is generated directly from the substrate types?
  • and mapping is ?

please make sure to clarify these items in a markdown file

[DylanVerstraete]

added in https://github.com/threefoldtech/tfchain_graphql/blob/8687ee51951423eee0bb38ebe81ba6bff67ac4eb/README.md#project-layout

[sameh-farouk]

Verified: Project structure was added to docs