Document the DMA Manager and OpenHCL memory model

[mattkur]

Memory model:

• VTL2 Memory allocation modes
• VTL2 MMIO space, and gap(s)
• Memory types (private pool, shared pool, etc.) + sizing heuristics (@mattkur to fill in docs on heuristics)
• How VTL2 accesses the VTL0 memory

DMA manager:

• Client (e.g. nvme, mana drivers) expectiations
• What it is, and general purpose