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.

.gitattributes

@@ -23,6 +23,9 @@ fixtures/** filter=lfs diff=lfs merge=lfs -text
 
 # The manual's pictures live in LFS for the same reason the models do: a
 # screenshot or a GIF changes wholesale when the interface it shows changes,
-# and every re-recording would otherwise stay in every clone for good. CI's
-# pulls exclude the directory; nothing built or tested reads it.
+# and every re-recording would otherwise stay in every clone for good. The
+# desktop and benchmark legs exclude the directory, since nothing they build
+# or test reads it; the Android and Windows legs fetch it, because the APK
+# and the installer carry the manual (docs/manual/index.html) with its
+# pictures, and their packagers refuse a pointer.
 docs/manual/media/** filter=lfs diff=lfs merge=lfs -text