Record the current AReno backend topology

[adohe]

Spec candidate: #9.

Scope

• Add a short architecture note describing Trainer -> Backend -> ArenoBackend -> ArenoEngine.
• State that the current runtime path is AReno-owned CUDA/NVIDIA code.
• State that ArenoBackend currently owns both rollout and train calls through one colocated engine.

Acceptance

• Cites the current files/symbols.
• Does not promise MLX, TPU, vLLM, SGLang, Megatron, or Ray integration.
• Verification: docs file renders as plain Markdown or RST.