Group migration MVC - User Documentation

User documentation proposal

We need to clearly document the GitLab Group Migration functionality available as of the completion of the MVC epic, as a distinct feature from the Group Import from an export file.

The feature should be labeled as beta, explicitly calling out the fact that it is bare-bones and may not be useful enough for most users. Please consider the example on this page for the warning box treatment: https://docs.gitlab.com/ee/development/bulk_import.html

This message should be used for the warning text:
"Caution: This feature is under development and currently migrates only some of the Group data. Please see below for the full list of what is included in the migration at this time."

The document should then clearly list which attributes of a Group are currently included in the migration, with an explicit note that anything not mentioned in the list is not included.