Change landing page of the API tab to point to the overview of the NSL module.

PiperOrigin-RevId: 267048890

Author: Neural-Link Team

g3doc/_book.yaml

@@ -28,8 +28,8 @@ upper_tabs:
     - name: API
       skip_translation: true
       contents:
+      - include: /neural_structured_learning/api_docs/python/_toc.yaml
       - title: All symbols
         path: /neural_structured_learning/api_docs/python/
-      - include: /neural_structured_learning/api_docs/python/_toc.yaml
 
 - include: /_upper_tabs_right.yaml