docs: keep only reference documentation, archive one-offs

docs/nextcloud-sync.md

@@ -2,7 +2,8 @@
 
 XAMXAM pushes its latest SQLite snapshot to a Nextcloud folder so that a
 server/disk failure doesn't take the backup history with it. This is the
-implementation of *Phase 5 — Remote Sync* from [backup-plan.md](backup-plan.md).
+implementation of *Phase 5 — Remote Sync* from the archived
+[`backup-plan.md`](archive/backup-plan.md).
 
 ## What it does