require redcarpet 3.1 for user-friendly anchors

with_toc_data was updated in vmg/redcarpet#186
to generate pretty ID attributes for Hn headers, which is what we use
for cross-linking

_config.yml

@@ -5,5 +5,7 @@ markdown: redcarpet
 encoding: utf-8
 redcarpet:
   extensions: ["no_intra_emphasis", "fenced_code_blocks", "autolink", "tables", "with_toc_data", "strikethrough", "lax_spacing", "space_after_headers", "superscript", "footnotes"]
+# with_toc_data requires redcarpet 3.1 to get
+# pretty ID attributes for Hn headers (https://github.com/vmg/redcarpet/pull/186)
 baseurl: /scala-ref-markdown/
 # permalink: /docs/:title.html