Docs: VR pet, its models, credits and changes outside Nix

docs/pet.md (usage, configuration, how it works), docs/pet-models.md
(the model spec), the README's feature line, options, changes outside
Nix and Credits (Toon Cat FREE and Tuxedo Cat, CC-BY 4.0; Quaternius
Shiba Inu and Fox, CC0), the cleanup artifact, the layout and runtime
names, and the template.

docs/ui-patches.md

@@ -47,15 +47,15 @@ and `sudo systemctl disable` them.
 
 [Dashboard windows](dashboard-windows.md),
 [Steam close button](steam-close-button.md),
-[window curvature](window-curvature.md) and
-[window control bar](window-control-bar.md) patch SteamVR's dashboard while
-it runs. What they have in common:
+[window curvature](window-curvature.md),
+[window control bar](window-control-bar.md) and the [VR pet](pet.md) patch
+SteamVR's dashboard while it runs. What they have in common:
 
 - Each turns on the [SteamVR debugger](steamvr-debugger.md) (**the first
   time, restart SteamVR once**).
 - They depend on SteamVR UI internals: each is found by signature (its
   entry in `modules/steam-ui-patches/lib/signatures.json` has the patch's
-  name); after an update that changes them the dashboard stays stock (see
+  name; the VR pet's are inline in `modules/pet/systemui.js`); after an update that changes them the dashboard stays stock (see
   [after a Steam update](#after-a-steam-update)). Tested with SteamVR build
   11008059.
 - All are laser-only: gamepad navigation sees the stock dashboard.
@@ -102,8 +102,8 @@ per such patch in `~/.local/state/steam-frame-nix/ui-patches/<name>.json`
   service writes the file atomically, only on change, only for that page's
   `state` patches, at most 64 KiB.
 
-Used by the [window control bar](window-control-bar.md) and the
-[Steam close button](steam-close-button.md). The file is user data, not
+Used by the [window control bar](window-control-bar.md), the
+[Steam close button](steam-close-button.md) and the [VR pet](pet.md). The file is user data, not
 generated by Nix: it is kept when the patch is disabled or removed and
 removed only by `steam-frame-nix-cleanup --all` or `install.sh uninstall`;
 see [changes outside Nix](../README.md#changes-outside-nix-exceptions).