[doc] Document in a CONTRIBUTING.md how hexa archi is structured, factory / manager works in edge_orchestrator

[bojeanson]

Description

In edge_orchestrator, we made some choice on how the repo is structurized:

• Document it
• Explain it (hexagonal archi): application, interface, domain, infrastructure
• Detail the part about interface (port), implementation (adapter), factory pattern and manager

Acceptance criteria

Better CONTRIBUTING rules