Add Group relations export API docs

doc/api/group_relations_export.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/#assignments
+---
+
+# Group Relations Export API
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/59978) in GitLab 13.12.
+
+With the Group Relations Export API, you can partially export group structure. This API is similar
+to [group export](group_import_export.md),
+but it exports each top-level relation (for example, milestones/boards/labels) as a separate file
+instead of one archive. The group relations export API is primarily used in [group migration](../user/group/index.md).
+
+## Schedule new export
+
+Start a new group relations export:
+
+```plaintext
+POST /groups/:id/export_relations
+```
+
+| Attribute | Type           | Required | Description                              |
+| --------- | -------------- | -------- | ---------------------------------------- |
+| `id`      | integer/string | yes      | ID of the group owned by the authenticated user. |
+
+```shell
+curl --request POST --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/groups/1/export_relations"
+```
+
+```json
+{
+  "message": "202 Accepted"
+}
+```
+
+## Export status
+
+View the status of the relations export:
+
+```plaintext
+GET /groups/:id/export_relations/status
+```
+
+| Attribute | Type           | Required | Description                              |
+| --------- | -------------- | -------- | ---------------------------------------- |
+| `id`      | integer/string | yes      | ID of the group owned by the authenticated user. |
+
+```shell
+curl --request GET --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/groups/1/export_relations/status"
+```
+
+The status can be one of the following:
+
+- `0`: `started`
+- `1`: `finished`
+- `-1`: `failed`
+
+- `0` - `started`
+- `1` - `finished`
+- `-1` - `failed`
+
+```json
+[
+  {
+    "relation": "badges",
+    "status": 1,
+    "error": null,
+    "updated_at": "2021-05-04T11:25:20.423Z"
+  },
+  {
+    "relation": "boards",
+    "status": 1,
+    "error": null,
+    "updated_at": "2021-05-04T11:25:20.085Z"
+  }
+]
+```
+
+## Export download
+
+Download the finished relations export:
+
+```plaintext
+GET /groups/:id/export_relations/download
+```
+
+| Attribute       | Type           | Required | Description                              |
+| --------------- | -------------- | -------- | ---------------------------------------- |
+| `id`            | integer/string | yes      | ID of the group owned by the authenticated user. |
+| `relation`      | string         | yes      | Name of the group top-level relation to download. |
+
+```shell
+curl --header "PRIVATE-TOKEN: <your_access_token>" --remote-header-name --remote-name "https://gitlab.example.com/api/v4/groups/1/export_relations/download?relation=labels"
+```
+
+```shell
+ls labels.ndjson.gz
+labels.ndjson.gz
+```

doc/api/groups.md

@@ -1257,7 +1257,9 @@ Read more in the [Group Badges](group_badges.md) documentation.
 
 ## Group Import/Export
 
-Read more in the [Group Import/Export](group_import_export.md) documentation.
+Read more in the [Group Import/Export](group_import_export.md)
+and [Group Relations Export](group_relations_export.md)
+documentation.
 
 ## Share Groups with Groups