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.

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