📝 Add user-friendly documentation for CinetPay adapter

✨ Context

EasySwitch already includes an integration for CinetPay (easyswitch.integrators.cinetpay).
However, the documentation is currently too technical and may not be user-friendly for developers who are new to EasySwitch or to payment integrations.

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

📌 What needs to be done

• Create a dedicated documentation page for CinetPayAdapter.
• 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
• Include practical use cases (e.g., integrating CinetPay for a local e-commerce app).
• Make it easy to read (target: developers with little or no prior knowledge of EasySwitch).

✅ Expected outcome

• A user-friendly documentation page available in the EasySwitch docs (docs/integrators/cinetpay.md).
• Clear examples and explanations that reduce the learning curve.
• Ensure that the documentation is helpful for both beginners and experienced developers.

💡 Notes

• You can look at the BaseAdapter documentation as a reference.
• Bonus: include a visual diagram showing the CinetPay payment flow inside EasySwitch.

🎉 Why this matters

CinetPay is a widely used aggregator in West Africa.
Improving its documentation will make EasySwitch more accessible to local developers and encourage adoption.

This issue is Hacktoberfest-friendly 🚀