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 b28ddfc (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/configuration.md

@@ -48,8 +48,8 @@ let you pick a mode or default to the device's display.)
 
 ## gamescope / session following (Linux, Bazzite/SteamOS)
 
-Two mutually-exclusive models for a Steam/gamescope box. See [Bazzite](/docs/bazzite) for the full
-picture.
+Two mutually-exclusive models for a Steam/gamescope box. See [Steam / gamescope](/docs/gamescope) for
+the full picture (and [Bazzite](/docs/bazzite) for that distro's specifics).
 
 | Setting | Values | Meaning |
 |---|---|---|
@@ -62,6 +62,8 @@ picture.
 
 ## Compositor-specific (Linux)
 
+See your desktop page ([KDE](/docs/kde), [GNOME](/docs/gnome)) for when to set these.
+
 > **Managing virtual displays** — keep-alive after disconnect, exclusive vs. extend, and (on
 > Windows/KDE) persistent per-client scaling — now has its own settings surface in the web console
 > and `display-settings.json`. See [Virtual displays](/docs/virtual-displays). The two