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/list_collections.md

@@ -0,0 +1,57 @@
+# List Collections
+
+Get all collections for the authenticated user.
+
+**Endpoint**: `GET /api/collections`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Query Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| limit | integer | No | Number of collections to return (default: 50) |
+| offset | integer | No | Number of collections to skip (default: 0) |
+
+## Response (200 OK)
+
+```json
+{
+  "collections": [
+    {
+      "id": "550e8400-e29b-41d4-a716-446655440000",
+      "name": "Science Fiction",
+      "description": "My sci-fi collection",
+      "color": "#FF5733",
+      "icon": "🚀",
+      "auto_assign_rules": [
+        {
+          "field": "genre",
+          "operator": "equals",
+          "value": "Science Fiction",
+          "priority": 1,
+          "enabled": true
+        }
+      ],
+      "view_settings": {
+        "kobo": {
+          "view_mode": "grid",
+          "sort_order": "name",
+          "items_per_page": 24
+        }
+      },
+      "created_at": "2026-02-01T10:00:00Z"
+    }
+  ],
+  "total": 1
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Authentication required |
+| 500 | Internal server error |
+
+## Try It Out