Update Group Import/Export documentation

The following discussions from !20353 (merged) should be addressed:

• @eread started a discussion:

  > [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:

  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:

  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:

  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.