📝 Add user-friendly documentation for Airtel Money adapter

✨ Context
EasySwitch includes an integration for Airtel Money(easyswitch.integrators.Airtel Money).
However, the documentation is currently too technical and may not be easily understandable for developers who are new to EasySwitch or payment aggregators.

We want to provide a clear, pedagogical, and comprehensive guide for using the Airtel Money adapter.

📌 What needs to be done

Create a dedicated documentation page for Airtel Money adapter.

Explain step by step how to configure and use it:

• How to set up credentials (api_key, api_secret, base_url, etc.)
• Sandbox vs Production environments
• Required fields for transactions
• Common pitfalls & error handling

Add examples (Python code snippets) for:

• Sending a payment
• Checking transaction status
• Handling webhooks (validation & parsing)
• Refunding a transaction

Provide practical examples (e.g., integrating Airtel Money for mobile money payments).

Ensure the documentation is simple, user-friendly, and easy to follow.

✅ Expected outcome

• A dedicated, user-friendly documentation page in the EasySwitch docs (docs/integrators/airtel_money.md).
• Clear examples and explanations that reduce onboarding friction.
• Helpful for both beginners and experienced developers.

💡 Notes

• Look at the BaseAdapter documentation as a model.
• Bonus: include a visual diagram showing the Airtel Money payment workflow in EasySwitch.

🎉 Why this matters
Airtel Money is a popular aggregator for mobile money and digital payments in Africa. and Asia
Improving its documentation will make EasySwitch more accessible to local developers, and foster wider adoption.

This issue is Hacktoberfest-friendly 🚀