Rename index files in administration docs - 3

doc/administration/admin_area.md

doc/administration/backup_restore/backup_gitlab.md

doc/administration/dedicated/architecture.md

doc/administration/geo/index.md

doc/administration/geo/replication/datatypes.md

doc/administration/geo/replication/multiple_servers.md

doc/administration/geo/replication/object_storage.md

doc/administration/gitaly/_index.md

doc/administration/gitaly/configure_gitaly.md

doc/administration/gitaly/gitaly_geo_capabilities.md

doc/administration/gitaly/kubernetes.md

doc/administration/gitaly/monitoring.md

doc/administration/gitaly/praefect.md

doc/administration/gitaly/troubleshooting_gitaly_cluster.md

doc/administration/gitlab_duo_self_hosted/_index.md

...stration/gitlab_duo_self_hosted/configure_duo_features.md

doc/administration/instance_limits.md

doc/administration/lfs/_index.md

doc/administration/monitoring/performance/performance_bar.md

doc/administration/object_storage.md