docs(kobo): mark native Kobo sync as coming soon

Native Kobo sync is implemented server-side but not yet supported on
real devices, so stop documenting it as a working feature.

- Rewrite kobo-setup.md as a coming-soon stub: point users to KOReader
  (which runs on Kobo hardware) as the supported path today, and list
  what native sync will deliver when released
- Add 'Coming Soon' status banners to the Kobo protocol spec, all five
  Kobo endpoint docs, and both API references, noting the endpoints are
  under active development and may change
- Tag the device shelf endpoints as pending native Kobo support

docs/developer/api/kobo/markup_sync.md

@@ -1,5 +1,7 @@
 # Markup Sync
 
+> **Status: Coming Soon** — Native Kobo sync is not yet supported on real devices; this endpoint is under active development and may change.
+
 Sync markup highlights and annotations from Kobo device.
 
 **Endpoint**: `POST /api/sync/kobo/markup`