docs: Fixing bad URL in libraries TOC

Also added the libraries sub-TOC to the "API Guides" TOC.

See first comment for doc stage location.

Change-Id: I06aac939e40ffad7888801ea01f6d4decd6eb4d1

docs/html/guide/_book.yaml

@@ -367,6 +367,8 @@ toc:
 
 - title: Libraries
   path: /topic/libraries/index.html
+  section:
+  - include: /topic/libraries/_book.yaml
 
 - title: Administration
   path: /guide/topics/admin/index.html

docs/html/topic/libraries/_book.yaml

@@ -8,7 +8,7 @@ toc:
     path: /topic/libraries/support-library/setup.html
 
 - title: Data Binding Library
-  path: /topic/libraries/data-binding/guide.html
+  path: /topic/libraries/data-binding/index.html
 
 - title: Testing Support Library
   path: /topic/libraries/testing-support-library/index.html