Ship the manual with the Arch package, the Windows installer and the APK

The rendered manual was in the repository and nowhere else, so an
installed application still had nothing to open.

Each packager now carries docs/manual/index.html and its pictures, to
where the application will look for them: /usr/share/darkroom/manual on
Arch, manual\ beside darkroom.exe on Windows (where the models already
are, and where dr_plat::system_data_dirs points), and assets/manual in
the APK, stored rather than deflated since a GIF or PNG is already
compressed. The manual is about 27 MB, which the APK and the installer
both grow by; the pictures are 1600x1100 screenshots and short GIFs,
and against an APK that already carries 170 MB of inference runtime and
70 MB of models they are not worth re-encoding for.

The pictures are LFS objects, so each packager refuses a pointer where a
picture should be, as it already does for the models: shipped, a pointer
is a manual of broken images that nothing reports. The Android and
Windows CI legs therefore fetch docs/manual/media, which they excluded
while nothing they built read it, and the installer smoke test checks
that the page and every picture were installed.

docs/dev/windows.md

@@ -285,6 +285,8 @@ $LOCALAPPDATA\Programs\DarkRoom\
     scrfd_500m_640.int8.onnx   scrfd_2.5g_640.int8.onnx   scrfd_10g_640.int8.onnx
     yolo26s-sem-ade20k.onnx   yolo26s-sem-ade20k.classes.json   categories.txt
     migan-512.onnx
+  manual\
+    index.html   media\   (the rendered manual and its pictures)
   LICENSE
   uninstall.exe
 ```