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/notes/update_note.md

@@ -0,0 +1,55 @@
+# Update Note
+
+Update an existing note.
+
+**Endpoint**: `PUT /api/media-items/notes/{note_id}`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| note_id | string | Yes | Note UUID |
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| content | string | No | Updated note content |
+| position | string | No | Updated location reference |
+
+### Example Request
+
+```json
+{
+  "content": "Updated note content",
+  "position": "epubcfi(/6/4/2:20)"
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "id": "uuid",
+  "media_item_id": "uuid",
+  "user_id": "uuid",
+  "content": "Updated note content",
+  "position": "epubcfi(/6/4/2:20)",
+  "updated_at": "2026-01-31T10:00:00Z"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid note data |
+| 401 | Invalid or expired token |
+| 403 | User does not own this note |
+| 404 | Note not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->