* Add NetBird VPN setup guide for PiKVM
Document the full setup process for running NetBird on PiKVM's
read-only filesystem, including overlayfs configuration, systemd
service modifications, device registration (setup key and SSO),
state persistence, and common troubleshooting steps.
* fix: replace curl|bash install with AUR package build
Address maintainer review: use git clone + makepkg under kvmd-webterm
to build netbird-bin from AUR instead of piping install script to sh.
Also replace overlayfs with tmpfs + bind mount since PiKVM's kernel
lacks the overlay module, and update all systemd references to use the
AUR package's template unit (netbird@.service).
Huy Nguyen