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/collections/update_collection.md

@@ -0,0 +1,63 @@
+# Update Collection
+
+Update collection details.
+
+**Endpoint**: `PUT /api/collections/{id}`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| id | string (UUID) | Yes | Collection UUID |
+
+## Request Body
+
+All fields are optional. Include only fields you want to update.
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| name | string | No | Collection name (max 255 chars) |
+| description | string | No | Collection description |
+| color | string | No | Hex color code (e.g., "#FF5733") |
+| icon | string | No | Emoji icon (e.g., "🚀", "📖") |
+| auto_assign_rules | array | No | Array of rule objects (replaces existing rules) |
+| view_settings | object | No | Per-device display preferences |
+
+### Example Request
+
+```json
+{
+  "name": "Sci-Fi Favorites",
+  "description": "Updated description",
+  "color": "#7aa2f7",
+  "icon": "🌟"
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "id": "550e8400-e29b-41d4-a716-446655440000",
+  "name": "Sci-Fi Favorites",
+  "description": "Updated description",
+  "color": "#7aa2f7",
+  "icon": "🌟",
+  "auto_assign_rules": [],
+  "view_settings": {},
+  "created_at": "2026-02-01T10:00:00Z"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid request (validation failed) |
+| 401 | Authentication required |
+| 404 | Collection not found |
+| 500 | Internal server error |
+
+## Try It Out