docs: add GitHub Actions logs guide

[michalmaj]

Summary

Adds a guide for reading GitHub Actions logs when CI fails.

Why

Beginners often struggle to understand failed CI runs. This guide explains how to find the failed job, failed step, and first useful error message without reading the entire log blindly.

Changes

• Adds docs/github_actions_logs.md
• Explains how to find failed CI logs in a pull request
• Explains each CI workflow step
• Shows common failure examples for dependency sync, Ruff, formatting, tests, and imports
• Explains how to reproduce CI failures locally
• Adds guidance for CI failures on main
• Adds a GitHub Actions logs 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.