[Improvement] Improve CLI help command

### What would you like to be improved?

The current Gravitino CLI help system has several critical issues:

1. Unclear Structure in Global Help Output
2. Unstructured and Unclear options
3. Lack of Semantic Description
4. Redundant and Misplaced Examples

design docs: https://docs.google.com/document/d/1294mJ--B7gGlr44tpKqK4tvQQbU7lN_u3BeLUyn3330/edit?usp=sharing

### How should we improve?

plz see doc.