Add developer docs for the batched background migration framework
Problem to solve
As part of our ongoing primary key conversions we've introduced a new framework for batched background migrations.
Some of this is documented on https://docs.gitlab.com/ee/user/admin_area/monitoring/background_migrations.html, but we don't have any developer docs yet describing how to use them.
Further details
One of the migration test helpers for batched background migrations is documented at https://docs.gitlab.com/ee/development/testing_guide/testing_migrations_guide.html#have_scheduled_batched_migration
Proposal
Extend https://docs.gitlab.com/ee/development/background_migrations.html with details on the new framework and examples on how to use it with the new helpers:
queue_batched_background_migration
(revert_)initialize_conversion_of_integer_to_bigint
(revert_)backfill_conversion_of_integer_to_bigint
Who can address the issue
Other links/references
Edited by Yannis Roussos