Bump the trunk major version to 13

and clear the release notes.

GitOrigin-RevId: 5369517d20dd362a178a1b2d6c398d8898ee4620
diff --git a/docs/conf.py b/docs/conf.py
index 6217ead..07ffba7 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -48,9 +48,9 @@
 # built documents.
 #
 # The short X.Y version.
-version = '12.0'
+version = '13.0'
 # The full version, including alpha/beta/rc tags.
-release = '12.0'
+release = '13.0'
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.