update documentation

[luigiberducci]

Remove deprecated sections:

• Usage/Customized Usage Example/Custom Map/Random Track Generator (Beta)
• Signature reset/step in Example Usage/Customized Usage Example/Multiple Agents
• Empty section on Reproducibility

Enhancements:

• Add Example for choice of dynamics model in Usage/Customized Usage Example/Changing Parameters in Vehicle Dynamics
• Add section on different observation spaces (e.g., kinematic, dynamic, custom features)
• Update section API Referece/What’s In an Observation
• Update README

See pull request #50

[luigiberducci]

Update docstrings (numpy style)

[hzheng40]

See pull request #120