Enable section-folding of table of content

weihanglo · web-flow · commit eaf891dfdabc · 2019-11-09T03:14:53.000Z
The section-folding configuration has landed in mdBook. Enable this would make sidebar a little bit clear.
diff --git a/book.toml b/book.toml
@@ -6,3 +6,6 @@ author = "The Rust Community"
 [output.html.playpen]
 editable = true
 editor = "ace"
+
+[output.html.fold]
+enable = true
