docs: add authentication and user management API endpoints

Phase 2 part 2: Split auth and user endpoints
- Authentication: register, login, refresh_token, logout
- Users: get_profile, update_profile, update_theme, change_password
- Each endpoint in separate markdown file
- Include request/response examples and error codes

docs/api/authentication/logout.md

@@ -0,0 +1,35 @@
+# Logout User
+
+Invalidate the current JWT token.
+
+**Endpoint**: `POST /api/auth/logout`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Request Headers
+
+| Header | Type | Required | Description |
+|--------|------|-----------|-------------|
+| Authorization | string | Yes | Bearer token (e.g., `Bearer eyJhbG...`) |
+
+### Example Request
+
+```http
+POST /api/auth/logout
+Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+```
+
+## Response (204 No Content)
+
+No response body.
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid or expired token |
+| 403 | Token already invalidated |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->