doc: plakar out of the current architecture, kept as reference

backup-strategy.md now leads with the three target legs (local incus
push to sdb, nuc pull over WireGuard, restic to S3 for DBs and
selected trees) and carries the plakar/ks2 story in a closing History
section. restic-backup.md, install.md, local-backup-cron.md and the
ks2 docs describe the predecessor only as 'the first S3
implementation', with one operational note that plakar is still
installed for issue #2338 reproduction.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ks4/local-backup-cron.md

@@ -15,9 +15,9 @@ backup of any kind was running). Overview of all legs:
   push to ks2; see [incus-copy.md](incus-copy.md)). An S3 leg for the
   instance filesystems was designed and shelved
   ([restic-backup.md](restic-backup.md) §6).
-- The rsync stopgap to ks2 (`incus-backup.sh`, 04:00) and the plakar
-  data leg (04:30) are both **retired**; S3 backups are restic's job
-  at 05:00.
+- The rsync stopgap to ks2 (`incus-backup.sh`, 04:00) and the first
+  S3 implementation (04:30) are both **retired**; S3 backups are
+  restic's job at 05:00 ([restic-backup.md](restic-backup.md)).
 
 ## Cleanup done the same day