docs: improve Code Style section in CONTRIBUTING.md

[yuchengpersonal]

Summary

This PR improves the Code Style section in CONTRIBUTING.md to make it more actionable for new contributors:

1. Specifies that Python code uses for linting and formatting, and provides the command to auto-fix pre-commit failures
2. Adds a relative link to for Markdown style rules
3. Adds a note explaining what pre-commit checks run automatically
4. Fixes the typo of 'well-documented' (was missing the trailing 'd')

Fixes #4421