Bump kramdown from 2.1.0 to 2.3.1

Bumps [kramdown](https://github.com/gettalong/kramdown) from 2.1.0 to 2.3.1.
- [Release notes](https://github.com/gettalong/kramdown/releases)
- [Changelog](https://github.com/gettalong/kramdown/blob/master/doc/news.page)
- [Commits](https://github.com/gettalong/kramdown/commits)

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -57,7 +57,8 @@ GEM
       html-pipeline (~> 2.2)
       jekyll (>= 3.0, < 5.0)
     json (2.2.0)
-    kramdown (2.1.0)
+    kramdown (2.3.1)
+      rexml
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)
     liquid (4.0.3)
@@ -76,6 +77,7 @@ GEM
     rb-fsevent (0.10.3)
     rb-inotify (0.10.0)
       ffi (~> 1.0)
+    rexml (3.2.4)
     rouge (3.10.0)
     ruby_dep (1.5.0)
     safe_yaml (1.0.5)