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/get_collection.md

@@ -0,0 +1,54 @@
+# Get Collection
+
+Get single collection with all books.
+
+**Endpoint**: `GET /api/collections/{id}`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| id | string (UUID) | Yes | Collection UUID |
+
+## Query Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| include_books | boolean | No | Include books in response (default: true) |
+| limit | integer | No | Number of books to return (default: 50) |
+| offset | integer | No | Number of books to skip (default: 0) |
+
+## Response (200 OK)
+
+```json
+{
+  "id": "550e8400-e29b-41d4-a716-446655440000",
+  "name": "Science Fiction",
+  "description": "My sci-fi collection",
+  "color": "#FF5733",
+  "icon": "🚀",
+  "books": [
+    {
+      "media_item_id": "660e8400-e29b-41d4-a716-446655440000",
+      "title": "Foundation",
+      "author": "Isaac Asimov",
+      "cover_image_path": "/covers/foundation.jpg"
+    }
+  ],
+  "auto_assign_rules": [],
+  "view_settings": {},
+  "created_at": "2026-02-01T10:00:00Z"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Authentication required |
+| 404 | Collection not found |
+| 500 | Internal server error |
+
+## Try It Out