[Feature Request]: Add migration guide from legacy library

### Preflight Checklist

- [x] I have read the [Contributing Guidelines](https://github.com/EnAccess/OpenPAYGO-python/blob/main/CONTRIBUTING.md) for this project, if it exists.
- [x] I agree to follow the [Code of Conduct](https://github.com/EnAccess/OpenPAYGO-python/blob/main/CODE_OF_CONDUCT.md) that this project adheres to.
- [x] I have searched the [issue tracker](https://github.com/EnAccess/OpenPAYGO-python/issues) for a feature request that matches the one I want to file, without success.

### Problem Description

Currently, it's not really clear how to migration from the legacy library, as this library uses a slightly different abstraction level in terms of Token and values.

### Proposed Solution

Add a migration guide.

### Alternatives Considered

N/A

### Additional Information

_No response_