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/register_device.md

@@ -0,0 +1,48 @@
+# Register Device
+
+Register a new device for sync.
+
+**Endpoint**: `POST /api/devices/register`
+**Auth**: Not required (device registration flow)
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| device_name | string | Yes | Device name |
+| device_type | string | Yes | Device type: kobo, koreader, web, mobile |
+| device_identifier | string | Yes | Hardware-specific ID |
+
+### Example Request
+
+```json
+{
+  "device_name": "My Kobo Clara",
+  "device_type": "kobo",
+  "device_identifier": "hardware-specific-id"
+}
+```
+
+## Response (201 Created)
+
+```json
+{
+  "device_id": "uuid",
+  "registration_id": "registration-uuid",
+  "auth_url": "https://bookhoard.com/devices/auth/confirm/abc123",
+  "qr_code": "data:image/png;base64,iVBORw0KG...",
+  "expires_in": 300
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid device data |
+| 409 | Device already registered |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->