Document how to pull registrar DS records from Knot on ns1

Explain that key material is auto-managed in the KASP keystore under
/var/lib/knot, and that the registrar DS is generated per zone with
`sudo -u knot keymgr <zone> ds`.

docs/src/dns.md

@@ -32,6 +32,30 @@ Automatic signing on `ns1` only, policy `cnx`: ECDSA P-256/SHA-256. The ZSK
 auto-rolls; the KSK is kept stable, so the DS at the registrar only changes on a
 manual KSK rollover.
 
+### Registrar DS records
+
+Knot manages all key material itself on `ns1` (the only signer); the KSK/ZSK
+private keys live in the KASP keystore under `/var/lib/knot` (backed up nightly —
+see [Backups](./backups.md)). You never touch the private keys directly.
+
+What a registrar needs is the **DS record** for a zone's KSK, which anchors the
+zone into the parent's chain of trust. Generate it on `ns1` — the `keymgr` wrapper
+is already pointed at Knot's config, and it runs as the `knot` user that owns the
+keystore:
+
+```
+sudo -u knot keymgr <zone> ds
+```
+
+e.g. `sudo -u knot keymgr cnx.email ds`. Paste the printed DS record (key tag,
+algorithm 13, digest type, digest) into the registrar's DNSSEC form for that
+domain. Repeat per signed zone (`cnx.network`, `buildfor.life`, `cnx.email`) at
+whichever registrar holds each delegation. After submitting, confirm the parent
+publishes it with `dig +short DS <zone>`.
+
+The ZSK rolls automatically and needs no registrar action; only a **KSK rollover**
+requires re-submitting the DS.
+
 > **Pending (manual):** submit DS records for `buildfor.life` and `cnx.email`
 > once they're at a DNSSEC-capable registrar.