docs: add user-friendly README with setup and usage instructions

[Paroksh1]

This PR adds a detailed and user-friendly README.md file to the Arithmetic Activity repository.

What's included:

1. Clear overview of the project and its purpose
2. Step-by-step installation and setup instructions
3. Usage guide for learners and educators
4. Instructions for contributors and translators
5. License and community acknowledgments

This aims to improve the onboarding experience for both users and contributors by making the project more accessible and understandable.

[quozl]

No thanks.

• it's wrong in several respects, you don't seem to know what you are talking about,
• other activities have used a conventional or standard format and there's no reason to make a new layout.