Create feature flag for incremental repository backups

Introducing incremental backups is risky and so will need some
development time to stabilise.

config/feature_flags/development/incremental_repository_backup.yml

+---
+name: incremental_repository_backup
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/79589
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/355945
+milestone: '14.9'
+type: development
+group: group::gitaly
+default_enabled: false

doc/raketasks/backup_restore.md

@@ -1854,3 +1854,22 @@ To enable it:
 ```ruby
 Feature.enable(:gitaly_backup)
 ```
+
+### Incremental repository backups
+
+> Introduced in GitLab 14.9 [with a flag](../administration/feature_flags.md) named `incremental_repository_backup`. Disabled by default.
+
+FLAG:
+On self-managed GitLab, by default this feature is not available. To make it available, ask an administrator to [enable the feature flag](../administration/feature_flags.md) named `incremental_repository_backup`.
+On GitLab.com, this feature is not available.
+This feature is not ready for production use.
+
+Incremental backups can be faster than full backups because they only pack changes since the last backup into the backup
+bundle for each repository. Because incremental backups require access to the previous backup, you can't use incremental
+backups with tar files.
+
+To create an incremental backup, run:
+
+```shell
+sudo gitlab-backup create SKIP=tar INCREMENTAL=yes
+```