Add support for migration in the code generation documentation

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 :)

Start by reviewing the existing code generation documentation and its surrounding migration guidance. Document a progressive migration path covering code organization, configuration placement, custom logic, waiting operations, overrides, and validation; done means provider maintainers can use the guidance to plan each migration step.

Written by the indexing model from the issue text.