fix: documentation inconsistencies

[rajephon]

Summary

This PR fixes two key documentation inconsistencies that could confuse users:

• Model name consistency: Updated documentation examples to use gpt-5-mini instead of gpt-4o-mini to match the model used in example.py and README.md
• Callback event accuracy: Corrected documentation to accurately reflect which callback events are supported by each streaming type, rather than incorrectly suggesting all types support the same events uniformly

Changes Made

Model Name Consistency

• Updated docs/index.md and docs/api/parser.md to use gpt-5-mini in code examples
• Ensures consistency across all documentation and example code

Callback Event Documentation

• Fixed inaccurate descriptions in README.md, docs/index.md, and docs/api/overview.md
• Added detailed explanations in docs/api/parser.md showing which events are supported by each type:
  • Object: on_start, on_complete, on_update
  • List: on_start, on_complete, on_append
  • String: on_start, on_complete, on_append
  • Atom: on_start, on_complete

Test plan

• All existing tests pass
• Documentation builds without errors
• Code examples in documentation are accurate and consistent

[dittos]

Thanks!