Add documentation about how to migrate a provider schema that got a `Computed` field

### Module version

```
❯ go list -m github.com/hashicorp/terraform-plugin-framework/...
github.com/hashicorp/terraform-plugin-framework v1.15.1
```

### Use-cases

As a provider maintainer I would like to setup a mux configuration to have a progressive migration from SDK resources to framework based resources.
One of the first step for setting up this migration is to setup a provider in the framework that got the same schemas that the provider in the SDK.
The main issue is that we got several fields that are string and Computed in our SDK configuration and there does not seem to be support for `Computed` attribute in a provider schema.

Even if we remove the `Computed` field from the SDK, it breaks most our resources.

### Attempted Solutions

Keeping the `Computed` field on but then the muxing does not work.
Removing the `Computed` and having all our test break. ([link to PR](https://github.com/scaleway/terraform-provider-scaleway/pull/3322))

### Proposal

Adding official documentation about how to migrate provider that have computed attributes.
It would be helpful to add examples and code snippet here.

### References

- https://developer.hashicorp.com/terraform/plugin/framework/migrating/attributes-blocks/fields
- https://github.com/hashicorp/terraform-plugin-framework/issues/510
- https://discuss.hashicorp.com/t/framework-migration-test-produces-non-empty-plan/54523/12
- https://github.com/scaleway/terraform-provider-scaleway/pull/3322