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/libraries/add_library_folder.md

@@ -0,0 +1,51 @@
+# Add Library Folder
+
+Add a folder to an existing library (Admin only).
+
+**Endpoint**: `POST /api/libraries/{library_id}/folders`
+**Auth**: Required (Admin only)
+**Content-Type**: `application/json`
+
+## Path Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| library_id | string | Yes | Library UUID |
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| folder_path | string | Yes | Absolute path to folder |
+
+### Example Request
+
+```json
+{
+  "folder_path": "/path/to/comics"
+}
+```
+
+## Response (201 Created)
+
+```json
+{
+  "id": "uuid",
+  "library_id": "uuid",
+  "folder_path": "/path/to/comics",
+  "created_at": "2026-01-31T10:00:00Z"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid folder path |
+| 401 | Invalid or expired token |
+| 403 | User is not an admin |
+| 404 | Library not found |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->