docs: dedicated Arch Linux host+client guide

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>

clients/decky/src/index.tsx

@@ -80,7 +80,7 @@ const QamPanel: FC = () => {
       {/* Pinned games — the "jump straight into Playnite" rows. Pin games from a host's
           picker (fullscreen page → host row → games button). */}
       {pins.pins.length > 0 && (
-        <PanelSection title="Games">
+        <PanelSection title="Pinned Games">
           {pins.pins.map((pin) => {
             const { online } = resolvePinHost(pin, hosts);
             return (