Enhance admin documentation related to Project-level Secure Files

Usage documentation for Project-level Secure Files is provided here, but details on the following topics are missing:

• backup of secure files from local storage
• migration of secure file objects from local to object storage and back
• how to configure secure files to be stored on object storage
• how to check integrity of secure files database against what is stored on disk

This is due to the associated functionality not having been deployed yet, as per the following issues:

• Secure Files are not included in backup
• Consolidated object storage configuration does not work for ci_secure_files (Secure Files)
• No rake task provided for migrating Secure Files from local to object storage and back
• Create integrity check for Secure Files objects

Once the functionality requested in these issues is delivered the documentation should be updated in the following areas (probably not an exhaustive list):

• https://docs.gitlab.com/ee/raketasks/backup_gitlab.html
  • add secure files to list of data types backed up
  • how to SKIP backup of secure files
• https://docs.gitlab.com/ee/administration/raketasks/check.html#uploaded-files-integrity
  • details of integrity check rake task
• new section under Administer docs covering rake task to migrate secure files to/from object storage (similar to https://docs.gitlab.com/ee/administration/raketasks/uploads/migrate.html)
• https://docs.gitlab.com/ee/administration/object_storage.html#configure-each-object-type-to-define-its-own-storage-connection-storage-specific-form
  • add entry for secure files
• https://docs.gitlab.com/ee/administration/object_storage.html#migrate-to-object-storage
  • add entry/link for secure files
• https://docs.gitlab.com/ee/administration/object_storage.html#configure-the-parameters-of-each-object
  • add entry/link for secure files
• https://docs.gitlab.com/charts/advanced/external-object-storage/
  • add entries for secure files