Fix kiwix ZIM volume mount to work with a symlink

Bind-mounting data/kiwix/ (the directory) meant a symlink placed
inside it broke: the container has to resolve that symlink itself,
in its own filesystem, where the host path it points to doesn't
exist. Binding the exact data/kiwix/wikipedia.zim file path instead
resolves it on the host at mount time (plain mount(2) semantics), so
the real ZIM can live anywhere on the host without duplicating a
multi-GB file into this repo's data/ directory.

docs/development.adoc

@@ -89,7 +89,14 @@ self-contained archive, obtain one locally:
   browsing images offline doesn't matter here.
 * Download it to `data/kiwix/wikipedia.zim` (the exact filename
   `docker-compose.yaml`'s `kiwix` service expects — rename after
-  downloading, Kiwix's own filenames are much more specific).
+  downloading, Kiwix's own filenames are much more specific). A symlink
+  there works too (`ln -s /wherever/the-real-file.zim
+  data/kiwix/wikipedia.zim`), so the real file doesn't have to sit inside
+  this repo's checkout at all — the compose file's volume binds that exact
+  path, not the `data/kiwix/` directory, specifically so this resolves
+  correctly (a symlink *inside* an already-mounted directory wouldn't: the
+  container would have to resolve it itself, in its own filesystem, where
+  the host path it points to doesn't exist).
 
 Once it's there, `docker compose up -d kiwix` (or a full restart)
 picks it up — `GET /api/kiwix` reports `available: true` and the