Bump kramdown from 1.17.0 to 2.3.0 in /site

Bumps [kramdown](https://github.com/gettalong/kramdown) from 1.17.0 to 2.3.0.
- [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>

site/Gemfile.lock

@@ -48,7 +48,8 @@ GEM
       jekyll (>= 3.3, < 5.0)
     jekyll-watch (2.2.1)
       listen (~> 3.0)
-    kramdown (1.17.0)
+    kramdown (2.3.0)
+      rexml
     liquid (4.0.3)
     listen (3.2.0)
       rb-fsevent (~> 0.10, >= 0.10.3)
@@ -61,6 +62,7 @@ GEM
     rb-inotify (0.10.0)
       ffi (~> 1.0)
     redcarpet (3.5.0)
+    rexml (3.2.4)
     rouge (3.13.0)
     safe_yaml (1.0.5)
     sass (3.7.4)