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/analytics/get_analytics.md

@@ -0,0 +1,49 @@
+# Get Reading Statistics
+
+Retrieve reading statistics for a date range.
+
+**Endpoint**: `GET /api/analytics/reading-stats`
+**Auth**: Required
+
+## Query Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| start_date | string | No | Start date (ISO 8601 format) |
+| end_date | string | No | End date (ISO 8601 format) |
+
+## Request Headers
+
+| Header | Type | Required | Description |
+|--------|------|-----------|-------------|
+| Authorization | string | Yes | Bearer token |
+
+### Example Request
+
+```http
+GET /api/analytics/reading-stats?start_date=2026-01-01&end_date=2026-01-31
+Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+```
+
+## Response (200 OK)
+
+```json
+{
+  "pages_read": 1250,
+  "books_completed": 5,
+  "reading_time_hours": 42.5,
+  "sessions_count": 28,
+  "average_session_minutes": 91
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid date format |
+| 401 | Invalid or expired token |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->