Use automatic TOC for rules.md

RELNOTES: None.
PiperOrigin-RevId: 221118548
diff --git a/site/_config.yml b/site/_config.yml
index b66b8da..a57a92b 100644
--- a/site/_config.yml
+++ b/site/_config.yml
@@ -2,6 +2,7 @@
 markdown: kramdown
 kramdown:
   syntax_highlighter: rouge
+  toc_levels: "2,3"
 highlighter: rouge
 
 paginate: 10