Update the doc of migration
Update the doc of migration

add the migration method after 1.8.x

Signed-off-by: DQ <5b460793db966fe84244ff3796dd28c1ec367157@vmware.com>

reasonerjt/harbor,reasonerjt/harbor,reasonerjt/harbor,wy65701436/harbor,wy65701436/harbor,reasonerjt/harbor,wy65701436/harbor,reasonerjt/harbor,reasonerjt/harbor,wy65701436/harbor,wy65701436/harbor,wy65701436/harbor