docs: restructure API docs

docs/api/training.md

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+# Training Models
+
+This guide covers how to configure and run training experiments for the Brittle Star project using Hydra-based configurations.
+
+## Configuration
+
+The project uses a modular configuration system powered by [Hydra](https://hydra.cc/). Instead of passing many command-line flags, you select and override configuration groups.
+
+### Creating a Custom Experiment
+
+1. **Create a new experiment file:**
+   Create a file at `configs/experiment/my_experiment.yaml`. You can copy an existing one as a template:
+   ```bash
+   cp configs/experiment/base.yaml configs/experiment/my_experiment.yaml
+   ```
+
+2. **Edit `configs/experiment/my_experiment.yaml`** to set your experiment parameters:
+   ```yaml
+   # @package _global_
+   experiment:
+     exp_name: "my_custom_run"
+     seed: 42
+   ```
+
+## Training Execution
+
+To start a training run with the default settings defined in `configs/main_config.yaml`:
+
+```bash
+uv run python scripts/train.py
+```
+
+### Using a Custom Experiment Configuration
+
+To run with your custom experiment file:
+
+```bash
+uv run python scripts/train.py experiment=my_experiment
+```
+
+### Command-Line Overrides
+
+You can override any parameter directly from the command line using Hydra's dot notation. This is useful for quick tests:
+
+```bash
+uv run python scripts/train.py ppo.learning_rate=0.001 ppo.num_envs=32 logging.track=true
+```
+
+For more details on tracking your experiments, see [Tracking & Monitoring](./tracking.md).