docs: restructure host setup by distro, configuration by compositor

Split the docs' single distro×desktop axis (ubuntu-gnome / ubuntu-kde / fedora-kde) into two,
which deduplicates the shared mechanics and scales to distros that run several desktops (Arch):

- Install the host — per distro/OS (ubuntu, fedora, arch, bazzite, steamos-host, windows-host):
  GPU driver + package + input group, then a canonical "Configure your desktop" funnel.
- Configure your desktop — per compositor (kde, gnome, gamescope, sway): host.env, compositor
  quirks, the headless session, and starting the host.

New shared web-console page (enable · login password · arm pairing) removes the console/password
block that was copy-pasted across all seven host pages. Merged ubuntu-gnome + ubuntu-kde into
ubuntu; renamed fedora-kde to fedora; kept bazzite and steamos-host as dedicated appliance guides
(trimmed of duplication). Moved the KWin headless session, the GNOME EGL/lock traps, and the
gamescope attach/managed model out of the distro pages onto their compositor pages.

Fixed while restructuring: distro-specific paths on kde (kde-desktop-setup.sh is Fedora/Bazzite-only;
the .deb ships host.env.kde under /usr/share/punktfunk-host), the interactive "start the host" step
that was lost in the merge, sway over-claiming Hyprland, and a pre-existing broken anchor in
how-it-works.

Removal of the three old pages was captured by the preceding commit 8ebb614 (a concurrent commit
swept up the staged git-rm); the net docs tree is correct. Fumadocs build + internal link/anchor
check green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs-site/content/docs/index.mdx

@@ -29,7 +29,7 @@ It's built for the things that make streaming feel native:
 <Cards>
   <Card title="How It Works" href="/docs/how-it-works" description="The ideas behind punktfunk in a few minutes — virtual displays, the two protocols, pairing." />
   <Card title="Quick Start" href="/docs/quickstart" description="From nothing to streaming: set up a host and connect your first client." />
-  <Card title="Host Setup" href="/docs/requirements" description="Install the host on Ubuntu (GNOME or KDE), Fedora (KDE), or Bazzite." />
+  <Card title="Host Setup" href="/docs/requirements" description="Install on Ubuntu, Fedora, Arch, Bazzite, SteamOS, or Windows." />
   <Card title="Connect a Client" href="/docs/clients" description="Stream with the native app for your device — macOS, Linux, Windows, Android — or any Moonlight client." />
   <Card title="API Reference" href="/api" description="Interactive OpenAPI reference for the host's management REST API — status, devices, pairing, library." />
 </Cards>
@@ -37,7 +37,7 @@ It's built for the things that make streaming feel native:
 ## What you need
 
 - A **host** with a supported GPU — either a **Linux** machine running one of the
-  [supported setups](/docs/requirements) (**Ubuntu** GNOME or KDE, **Fedora** KDE, or **Bazzite**), or
+  [supported setups](/docs/requirements) (**Ubuntu**, **Fedora**, **Arch**, or **Bazzite**), or
   a **[Windows](/docs/windows-host) PC**.
 - A **client device** to stream to — there are native apps for **macOS, iOS/iPadOS, tvOS, Linux,
   Windows, and Android**, plus any device that runs **Moonlight**.