Update BG migration docs for removed methods

What does this MR do?

Remove documentation that references background migration helper methods we removed in !73306 (merged).

We now only have two migration helpers for enqueuing background migrations, the most commonly used queue_background_migration_jobs_by_range_at_intervals and migrate_in.

I also updated the migration example to be compatible with the use of the helpers we support and recommend now. We also renamed the model and table that were used in that example, so I fixed those to make it less confusing.

Related issues

Related to #343055 (closed)

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