docs: second BC example (Inventory or Billing)

[joao-oliveira-softtor]

Track

Good first issue — see ROADMAP

Goal

Add examples/<bc>/ mirroring the Order bounded context so contributors see a second vertical slice (state machine VO, cross-BC listener, or read model).

Acceptance

• Layers: domain → application → infrastructure → presentation
• README explaining differences vs Order
• Follows AggregateRoot / pure repo / EventPublisher-in-Handler rules