Merge pull request rust-lang#1988 from rust-lang/TC/enable-chapter-folding

Enable folding of chapter listing in navigation sidebar

Author: ehuss

book.toml

@@ -10,6 +10,10 @@ git-repository-url = "https://github.com/rust-lang/reference/"
 edit-url-template = "https://github.com/rust-lang/reference/edit/master/{path}"
 smart-punctuation = true
 
+[output.html.fold]
+enable = true
+level = 0
+
 [output.html.search.chapter]
 "test-summary.md" = { enable = false }
 "grammar.md" = { enable = false }