Merge branch 'kassio/bulkimport-documentation' into 'master'

Add initial documentation about GitLab Migration

See merge request gitlab-org/gitlab!48082

doc/development/bulk_import.md

+---
+stage: Manage
+group: Import
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
+---
+
+# GitLab Group Migration
+
+[Introduced](https://gitlab.com/groups/gitlab-org/-/epics/2771) in GitLab 13.7.
+
+CAUTION: **Caution:**
+This feature is [under construction](https://gitlab.com/groups/gitlab-org/-/epics/2771) and its API/Architecture might change in the future.
+
+GitLab Group Migration is the evolution of Project and Group Import functionality. The
+goal is to have an easier way to the user migrate a whole Group, including
+Projects, from one GitLab instance to another.
+
+## Design decisions
+
+### Overview
+
+The following architectural diagram illustrates how the Group Migration
+works with a set of [ETL](#etl) Pipelines leveraging from the current [GitLab APIs](#api).
+
+![Simplified Component Overview](img/bulk_imports_overview_v13_7.png)
+
+### [ETL](https://www.ibm.com/cloud/learn/etl)
+
+<!-- Direct quote from the IBM URL link -->
+
+> ETL, for extract, transform and load, is a data integration process that
+> combines data from multiple data sources into a single, consistent data store
+> that is loaded into a data warehouse or other target system.
+
+Using ETL architecture makes the code more explicit and easier to follow, test and extend. The
+idea is to have one ETL pipeline for each relation to be imported.
+
+### API
+
+The current [Project](../user/project/settings/import_export.md) and [Group](../user/group/settings/import_export.md) Import are file based, so they require an export
+step to generate the file to be imported.
+
+GitLab Group migration leverages on [GitLab API](../api/README.md) to speed the migration.
+
+And, because we're on the road to [GraphQL](../api/README.md#road-to-graphql),
+GitLab Group Migration will be contributing towards to expand the GraphQL API coverage, which benefits both GitLab
+and its users.
+
+### Namespace
+
+The migration process starts with the creation of a [`BulkImport`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/models/bulk_import.rb)
+record to keep track of the migration. From there all the code related to the
+GitLab Group Migration can be found under the new `BulkImports` namespace in all the application layers.