docs: add example pull request

[michalmaj]

Summary

Adds a guide page showing what a clear pull request looks like.

Why

The repository teaches a branch and pull request based workflow. A concrete example helps readers understand how to write useful pull request titles, descriptions, change summaries, and notes.

Changes

• Adds docs/example_pull_request.md
• Shows an example pull request title
• Shows a complete example pull request description
• Explains the purpose of Summary, Why, Changes, Type of change, Checklist, and Notes sections
• Compares good and bad pull request examples
• Adds guidance for self-review before opening a pull request
• Adds an example pull request 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.