docs: Creating new support library revision history page

Previously, the support library revision history had been on
index.html, along with the overview. Pulling the history out onto
its own page. (There doesn't seem to be a way to set up a redirect--
redirects from /foo.html#bar to /bar.html don't seem to be honored.)

bug: 28721552
Change-Id: I26b8aead4161f1555cead02a4c61cc1b45ca1dbe

docs/html/topic/libraries/_book.yaml

@@ -6,6 +6,8 @@ toc:
     path: /topic/libraries/support-library/features.html
   - title: Setup
     path: /topic/libraries/support-library/setup.html
+  - title: Revision History
+    path: /topic/libraries/support-library/revisions.html
 
 - title: Data Binding Library
   path: /topic/libraries/data-binding/index.html