Add documentation on how to add new migrations

We already have information on how to run migrations at https://kernelci.org/docs/api/api-details/#migrations.
It is useful to add information for adding new migrations for developers. We could add a separate documentation file under `doc/` specifically for developer information.

Read the existing migration guidance at https://kernelci.org/docs/api/api-details/#migrations, then inspect the `doc/` directory to understand its structure. Add a separate developer-focused documentation file explaining how to add new migrations, and verify that it is discoverable alongside the existing migration information.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.