Feature/update doc links response error format

[eddyjaga]

Description

This pull request primarily updates documentation and error handling in the codebase. The main changes include fixing broken documentation links to Zai's onboarding guides by updating their URLs throughout the project, and improving error message formatting in API responses to better handle different error structures.

Documentation link updates:

• Updated all references to Zai's onboarding documentation URLs from /onboarding-a-payin-user and /onboarding-a-payout-user to /onboarding-a-pay-in-user and /onboarding-a-pay-out-user in README.md, docs/USERS.md, docs/USER_ID_FIELD.md, docs/USER_QUICK_REFERENCE.md, IMPLEMENTATION.md, and lib/zai_payment/resources/user.rb to fix broken links and ensure consistency. [1] [2] [3] [4] [5] [6] [7]

Error handling improvements:

• Enhanced the error message extraction logic in lib/zai_payment/response.rb to use a new format_errors method, which formats errors from arrays, hashes, or strings for clearer API error reporting.

Project management:

• Simplified the pull request template in .github/PULL_REQUEST_TEMPLATE.md by removing checklists and sections, focusing on a short description and testing notes.