docs: add rootless mode cross-reference to userns-remap page (fixes #12171)

The userns-remap page had no mention of rootless mode, so users didn't
know about the alternative. Added a note clarifying that userns-remap
still runs the daemon as root, with a link to rootless mode. The
rootless page already links back to userns-remap.

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

Author: dvdksn

content/manuals/engine/security/userns-remap.md

@@ -16,6 +16,12 @@ can re-map this user to a less-privileged user on the Docker host. The mapped
 user is assigned a range of UIDs which function within the namespace as normal
 UIDs from 0 to 65536, but have no privileges on the host machine itself.
 
+> [!NOTE]
+>
+> With `userns-remap`, the Docker daemon still runs as root. To run both the
+> daemon and containers without root privileges, see [Rootless mode](rootless/_index.md)
+> instead.
+
 ## About remapping and subordinate user and group IDs
 
 The remapping itself is handled by two files: `/etc/subuid` and `/etc/subgid`.