docs: add common questions

[michalmaj]

Summary

Adds a Common questions page and links it from the README.

Why

Readers may have practical questions about the project decisions before reading the full guide. A focused FAQ makes the repository easier to understand for both beginners and more experienced Python developers.

Changes

• Adds docs/common_questions.md
• Explains why the guide uses uv
• Explains why uv.lock is committed
• Explains why the project uses a src/ layout
• Clarifies that this is not yet a full packaging guide
• Explains pythonpath = ["src"]
• Explains Ruff, ignored rules, CI, and uv sync --locked
• Adds a Common questions link to the README

Type of change

• Documentation
• Project configuration
• New feature
• Tests
• CI/CD
• Refactoring
• Bug fix

Checklist

• The change is small and focused
• The pull request title is clear
• I updated documentation if needed
• I added or updated tests if needed
• I ran local checks if applicable

Notes

Documentation-only change.