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/meta.json

@@ -5,27 +5,31 @@
     "how-it-works",
     "security",
     "quickstart",
-    "install",
-    "---Host Setup---",
+    "---Install the host---",
     "requirements",
-    "ubuntu-gnome",
-    "ubuntu-kde",
-    "fedora-kde",
+    "install",
+    "ubuntu",
+    "fedora",
     "arch",
     "bazzite",
     "steamos-host",
     "windows-host",
+    "web-console",
+    "---Configure your desktop---",
+    "configuration",
+    "kde",
+    "gnome",
+    "gamescope",
+    "sway",
     "running-as-a-service",
+    "virtual-displays",
+    "host-cli",
     "---Connecting---",
     "clients",
     "install-client",
     "steam-deck",
     "moonlight",
     "pairing",
-    "---Configuration---",
-    "configuration",
-    "virtual-displays",
-    "host-cli",
     "---Troubleshooting---",
     "troubleshooting",
     "stats",