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/book-matching/search_books.md

@@ -0,0 +1,56 @@
+# Search Books for Matching
+
+Query books to find potential matches for linking.
+
+**Endpoint**: `POST /api/sync/books/query`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| identifiers | array | No | List of identifiers (ISBN, UUID) |
+| sha256 | string | No | SHA256 hash of book file |
+| title | string | No | Book title |
+| author | string | No | Book author |
+| file_size | integer | No | File size in bytes |
+
+### Example Request
+
+```json
+{
+  "identifiers": ["isbn:978-0345391802", "uuid:abc-123"],
+  "sha256": "a1b2c3d4e5f6abc123...",
+  "title": "The Hobbit",
+  "author": "J.R.R. Tolkien",
+  "file_size": 2456789
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "matches": [
+    {
+      "media_item_id": "uuid-123",
+      "bookhoard_uuid": "uuid-123",
+      "confidence": 1.0,
+      "match_method": "uuid_match"
+    }
+  ],
+  "action": "auto_link"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid query parameters |
+| 401 | Invalid or expired token |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->