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/ratings/create_rating.md

@@ -0,0 +1,52 @@
+# Create Rating
+
+Set a rating for a media item. Creates a new rating or updates an existing one.
+
+**Endpoint**: `POST /api/media-items/{media_id}/rating`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| media_id | string | Yes | Media item UUID |
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| rating | integer | Yes | Rating from 1-10 |
+
+### Example Request
+
+```json
+{
+  "rating": 8
+}
+```
+
+## Response (201 Created)
+
+```json
+{
+  "rating": 8,
+  "user_id": "uuid",
+  "media_item_id": "uuid",
+  "created_at": "2026-01-31T10:00:00Z"
+}
+```
+
+**Rating Scale**: 1-10 (odd numbers = half-stars: 1=0.5★, 2=1★, 3=1.5★, ..., 9=4.5★, 10=5★)
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid rating (must be 1-10) |
+| 401 | Invalid or expired token |
+| 404 | Media item not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->

docs/developer/api/ratings/get_ratings.md

@@ -0,0 +1,48 @@
+# Get Rating
+
+Retrieve the current user's rating for a media item.
+
+**Endpoint**: `GET /api/media-items/{media_id}/rating`
+**Auth**: Required
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| media_id | string | Yes | Media item UUID |
+
+## Request Headers
+
+| Header | Type | Required | Description |
+|--------|------|-----------|-------------|
+| Authorization | string | Yes | Bearer token |
+
+### Example Request
+
+```http
+GET /api/media-items/uuid/rating
+Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+```
+
+## Response (200 OK)
+
+```json
+{
+  "rating": 8,
+  "user_id": "uuid",
+  "media_item_id": "uuid"
+}
+```
+
+**Rating Scale**: 1-10 (odd numbers = half-stars: 1=0.5★, 2=1★, 3=1.5★, ..., 9=4.5★, 10=5★)
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid or expired token |
+| 404 | Media item not found or no rating set |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->