docs: add Paystack Adapter usage example

[codefromlani]

This PR adds usage guide for the PaystackAdapter, demonstrating how to initialize payments, check transaction status, and issue refunds using EasySwitch.

The example includes:

• Configuration via EasySwitch.from_dict()
• Payment initialization flow
• Status checks and refunds
• Troubleshooting section for common API errors

Notes

• The examples was validated using a real Paystack test key (sk_test_...) and a sandbox transaction.
• Additionally, i updated the send_payment and refund method to use json_data instead of json for compatibility with the HTTPClient requests.

[Einswilli]

Good job @codefromlani !
Can w have a preview link?

[codefromlani]

Good job @codefromlani ! Can w have a preview link?

Thanks.
preview link