Clear information on differences in what relations are imported from version to version

We seem to receive many support questions about why certain relations can fail to import. Often the source instance will be an older version of GitLab, more than two minor releases older, with different Group or Project import_export.yml files.

To avoid this confusion it would be good to clearly explain where the relations that will be imported are configured, and provide links to the historical versions of these import_export.yml files, and any work-arounds we know about that might be helpful to Users who encounter issues when importing from older versions.