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/bulk_assign.md

@@ -0,0 +1,52 @@
+# Bulk Assign Books
+
+Add multiple books to a collection at once.
+
+**Endpoint**: `POST /api/collections/{id}/books`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| id | string (UUID) | Yes | Collection UUID |
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| book_ids | array of UUID | Yes | Array of book IDs to add |
+
+### Example Request
+
+```json
+{
+  "book_ids": [
+    "660e8400-e29b-41d4-a716-446655440000",
+    "770e8400-e29b-41d4-a716-446655440000",
+    "880e8400-e29b-41d4-a716-446655440000"
+  ]
+}
+```
+
+## Response (204 No Content)
+
+Books added to collection successfully. No response body.
+
+## Notes
+
+- Books already in the collection are ignored
+- This is more efficient than adding books individually
+- Maximum 1000 books per request
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid request (validation failed) |
+| 401 | Authentication required |
+| 404 | Collection or book(s) not found |
+| 500 | Internal server error |
+
+## Try It Out