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>

docs-site/content/docs/clients.md

@@ -47,7 +47,7 @@ It ships as a real package, not just a source build — full steps in
   `flatpak update`; this is also what the [Decky plugin](/docs/steam-deck) launches.
 - **Ubuntu / Debian** — `apt install punktfunk-client` from the punktfunk apt registry.
 - **Fedora / Bazzite** — `rpm-ostree install punktfunk-client` from the Gitea RPM registry.
-- **Arch / SteamOS** — the `punktfunk-client` split package from the `PKGBUILD`.
+- **Arch** — `sudo pacman -Sy punktfunk-client` from the signed binary repo (see [Arch Linux](/docs/arch)).
 
 Launch it, pick your host from the list, and stream. For scripting you can skip the host list and
 connect straight away: