Add support for migration in the code generation documentation

[remyleone]

As a provider maintainer I got existing provider code. I would like to migrate progressively to the new code generation tool that is offered and based on openapi. Could you add content about how to migrate progressively to this new way of writing code?

How can I organize my code to make this migration as easy as possible? Where should the code generation configuration code live? How can I manage custom logic? How can I handle waiting operation when an operation in ongoing? How can I handle override and validation?

This content could help a lot to migrate to this new code generation phase of terraform plugin development :)

[austinvalle]

Thanks @remyleone! I think falls into a nice "best practices + guidance" area of documentation. May not be something we get to immediately, but adding it to the list! 👍🏻