docs(api): document device sidecar endpoints; fix registration docs

- New get_sidecar_config.md for GET /api/devices/:id/sidecar and
  /sidecar/download: the .bookhoard.json config served to devices
  (endpoints, books keyed by per-format SHA-256 with UUID fallback,
  collections, format availability) used by the KOReader plugin to
  self-configure
- register_device.md: correct the response — no device_id at
  registration; auth_url is /devices/approve/:id (was the nonexistent
  /devices/auth/confirm/:id); document poll_interval and
  setup_instructions, and the approve-then-poll flow
- get_devices.md: fix the status endpoint path to
  POST /api/devices/register/status (was /api/devices/auth/status)

docs/developer/api/devices/get_devices.md

@@ -2,7 +2,7 @@
 
 Check device registration status or get device details.
 
-**Endpoint**: `POST /api/devices/auth/status` or `GET /api/devices/{device_id}`
+**Endpoint**: `POST /api/devices/register/status` or `GET /api/devices/{device_id}`
 **Auth**: Not required for status check, Required for device details
 **Content-Type**: `application/json` (for status check)