Update docs

Created by: matofeder

Update project docs and document the following (if there are some missing points in the official Harbor tutorials):

• Harbor - backup and restore, addressed by https://github.com/SovereignCloudStack/k8s-harbor/pull/28
• Harbor - persistency, addressed by https://github.com/SovereignCloudStack/k8s-harbor/pull/28
• Harbor - migration
• Harbor - upgrade