docs: restructure API docs

docs/api/simulation.md

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+# Simulation & Evaluation
+
+The simulation pipeline allows you to visualize trained models and evaluate their performance under various conditions.
+
+## Overview
+
+The simulation pipeline is metadata-driven. Training-specific configurations (morphology, arena, environment, etc.) are automatically loaded from the `_metadata.yaml` file associated with the model checkpoint.
+
+## Basic Simulation
+
+To simulate a model in the MuJoCo viewer:
+
+```bash
+uv run scripts/simulate.py simulation.model_path=runs/your_run/final_model.flax
+```
+
+## Amputation & Morphology Overrides
+
+You can test trained models on different morphologies (e.g., amputating legs) by providing a morphology override. The observations will be automatically padded up to the training morphology's dimensions:
+
+```bash
+uv run scripts/simulate.py \
+    simulation.model_path=runs/your_run/final_model.flax \
+    simulation.morphology_override=configs/morphology/3_arms.yaml
+```
+
+## Video Recording
+
+Recording videos requires the `[evaluation]` extra:
+
+```bash
+uv run scripts/simulate.py \
+    simulation.model_path=runs/your_run/final_model.flax \
+    simulation.record_video=true \
+    simulation.max_steps=1000
+```
+
+Videos and evaluation metadata are stored in timestamped folders alongside the model:
+`runs/your_run/final_model_evaluations/eval_<timestamp>/simulation.mp4`