Roadmap to v0.4.0

[Jiayuan-Gu]

v0.4.0 will include new features and several improvements:

• Vectorized environment optimized for visual observations
• Tutorials
• Documentations

The estimated release date is mid-January. We will also improve the management of assets and demonstrations, and provide a pip wheel for easier installation. Besides, we are making efforts to improve our ray-tracing backend Kuafu.