Expand 'The Model' docs in Wiki

[dgor82]

• [ ] Core domain model (LiveEvents, SoA, Facilities etc. )
  • [ ] Document esp. use of generic type parameter iTrade for matrix-like 2D domain model
• [ ] TradeModels
• [ ] Workflows in general (not all the specific ones)
  • [ ] And the derived_workflow_bridges model in particular.