Move the NFS docs to a new location

doc/administration/geo/replication/multiple_servers.md

doc/administration/gitaly/index.md

doc/administration/high_availability/nfs.md

...administration/high_availability/nfs_host_client_setup.md

doc/administration/monitoring/performance/performance_bar.md

doc/administration/nfs.md

doc/administration/pages/index.md

doc/administration/redis/replication_and_failover.md

doc/administration/reference_architectures/2k_users.md

doc/administration/reference_architectures/3k_users.md

doc/administration/reference_architectures/5k_users.md

...administration/reference_architectures/troubleshooting.md

doc/administration/repository_storage_paths.md

doc/install/aws/index.md

doc/install/requirements.md

doc/raketasks/backup_restore.md