Every other distro has a full Host Setup page; Arch only had table rows. Add
docs/arch.md (signed pacman binary repo: key import + repo + install, GPU
prereqs, service/linger, web console, client, PKGBUILD appendix), slot it into
the nav after fedora-kde, and point the install/client tables at it. Update the
client-install rows from 'from the PKGBUILD' to the binary repo now that it exists.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
