docs: make Code Style section actionable (closes #4421)

[Jah-yee]

Summary

This PR makes the 'Code Style' section in CONTRIBUTING.md actionable by replacing vague guidance with concrete, runnable commands.

Before

• Python: Follow existing code conventions. Pre-commit hooks enforce formatting.
• Markdown: Follow markdownlint rules (see .markdownlint.json).
• Keep functions focused and well-documented.

After

• Python: We use ruff for formatting and linting. Run uv run ruff check . to check your code and uv run ruff format . to auto-fix formatting issues before committing. Pre-commit hooks enforce these checks.
• Markdown: Follow markdownlint rules (see .markdownlint.json). Run uv run markdownlint docs/ to check.
• Keep functions focused and well-documented — aim for functions under 50 lines with a docstring explaining purpose, args, and return value.

Why

The original guidance was vague and not actionable for new contributors. By specifying the exact tools (ruff, markdownlint) and the exact commands to run, new contributors can immediately validate their code style before opening a PR.

Closes #4421