docs: add docs README landing/starting point

docs/README.md

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+# Documentation
+
+## Design & architecture ([`/design`](./design/))
+
+- [Controllers](./design/controllers.md): Macroscopig brain toplogy, centralized, arm-level, segment-level.
+- [MLP architecture](./design/mlp_architecture.md): Module wiring, state/action spaces, and actor-critic pipeline.
+- [Communication](./design/communication.md): Message propagation, Nerve-Net style.
+- [Learning algorithm](./design/learning_algorithm.md): RL techniques, i.e. PPO.
+- [Reward function](./design/learning_algorithm.md): Goals, fitness tracking, and reward structures.
+
+## API reference ([`/api`](./api/))
+
+- [Environment](./api/environment.md): MuJoCo environment interaction, state retrieval, and configuration.
+- [Simulate](./api/simulate.md): Simulation rendering.