Upgrade to kramdown 2.3.1 (#1169)

Merging this PR despite CI failure. The CI failure concerns building the reference manual, which fails because of change of `basictex` versions. The change has recently been reflected in Homebrew/homebrew-cask#103001, but evidently some brew/cask/basictex/etc. release/synchronization still needs to happen before we get to reap the benefit of this fix.

docs/Gemfile

@@ -19,7 +19,7 @@ gem "github-pages", "~> 209", group: :jekyll_plugins
 #end
 
 gem 'jekyll-numbered-headings'
-gem "kramdown", ">= 2.3.0"
+gem "kramdown", ">= 2.3.1"
 
 # Windows and JRuby does not include zoneinfo files, so bundle the tzinfo-data gem
 # and associated library.