BulkImports: Update contributor documentation
Problem to solve
With the introduction of the concurrent model for the Gitlab Group Migration, the current developer documentation (https://docs.gitlab.com/ee/development/bulk_import.html) is outdated.
- The epic description contains most of the information required to update the documentation: &5544
- Take in consideration concerns about future changes: !56242 (comment 530370120)
Edited by Kassio Borges