docs: restructure documentation into audience-based portals

BREAKING CHANGE: Documentation URLs have changed

New structure:
- user/ - End-user documentation (device setup, sync guides, frontend)
- developer/ - Developer documentation (API reference, protocols, specs)
- operations/ - Operations documentation (deployment, troubleshooting)
- contributing/ - Contribution guides

Changes:
- Created portal INDEX.md files for each audience section
- Moved device guides to user/devices/ (kobo-setup.md, koreader-setup.md)
- Moved API docs to developer/ (api-reference.md, collections-api.md)
- Moved sync guide to user/sync-guide.md
- Moved troubleshooting to operations/troubleshooting.md
- Moved all split API docs to developer/api/
- Renamed protocol files (kobo-protocol.md, koreader-protocol.md)
- Added placeholder user guides (frontend, user-areas, settings, admin)
- Updated all internal links to new paths
- Updated Go code (http_handler.go, navigation.go) for new paths
- Updated main INDEX.md for audience-based navigation

Benefits:
- Clear separation of user and developer documentation
- Scalable structure for future user guide expansion
- Better organization and discoverability
- Audience-specific landing pages

Related to DOCS_IMPLEMENTATION_PLAN.md Phase 2 completion

docs/developer/api/devices/get_devices.md

@@ -0,0 +1,62 @@
+# Get Device / Check Registration Status
+
+Check device registration status or get device details.
+
+**Endpoint**: `POST /api/devices/auth/status` or `GET /api/devices/{device_id}`
+**Auth**: Not required for status check, Required for device details
+**Content-Type**: `application/json` (for status check)
+
+## Request Body (Status Check)
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| registration_id | string | Yes | Registration UUID |
+
+### Example Request (Status Check)
+
+```json
+{
+  "registration_id": "registration-uuid"
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "status": "pending|approved|expired",
+  "auth_token": "device-bearer-token...",
+  "device_id": "uuid",
+  "sync_endpoints": {
+    "progress": "https://bookhoard.com/api/sync/progress",
+    "metadata": "https://bookhoard.com/api/sync/metadata",
+    "annotations": "https://bookhoard.com/api/sync/annotations"
+  }
+}
+```
+
+## Response (200 OK) - Device Details
+
+```json
+{
+  "id": "uuid",
+  "device_name": "My Kobo Clara",
+  "device_type": "kobo",
+  "last_sync": "2026-01-31T10:00:00Z",
+  "last_seen": "2026-01-31T10:05:00Z",
+  "sync_enabled": true,
+  "auto_sync": true,
+  "sync_frequency_minutes": 5
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid or expired token (for device details) |
+| 404 | Device or registration not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->