Update Group Import/Export documentation
The following discussions from !20353 should be addressed:
- [ ] @eread started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/20353#note_285945466):
> ```suggestion:-0+0
> > [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/20353) in GitLab 12.8 as an experimental feature. May change in future releases.
> ```
>
> Even better, the issue.
- [ ] @eread started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/20353#note_285945467):
> ```suggestion:-0+2
> Group Import/Export allows for exporting group structure and importing it at a new location.
> Used with [Project Import/Export](project_import_export.md), you can preserve connections with
> group-level relationships. For example, connection between a project issue and group epic.
> ```
- [ ] @eread started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/20353#note_285945469):
> ```suggestion:-0+7
> Group exports include the following:
>
> - Group milestones
> - Group boards
> - Group labels
> - Group badges
> - Group members
> - Sub-groups. Each sub-group includes all data above
> ```
- [ ] @eread started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/20353#note_285945470):
> ```suggestion:-0+0
> Time spent on exporting a group may vary depending on a size of the group. This endpoint
> returns either:
>
> - The exported archive when available.
> - 404 is returned otherwise.
> ```
issue