Bump the trunk major version to 13

and clear the release notes.
diff --git a/libcxx/docs/conf.py b/libcxx/docs/conf.py
index 95cd98b..c58b187 100644
--- a/libcxx/docs/conf.py
+++ b/libcxx/docs/conf.py
@@ -47,9 +47,9 @@
 # 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.