Bump kramdown from 2.1.0 to 2.4.0 in /docs/static_site/src

Bumps [kramdown](https://github.com/gettalong/kramdown) from 2.1.0 to 2.4.0.
- [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)

---
updated-dependencies:
- dependency-name: kramdown
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>
diff --git a/docs/static_site/src/Gemfile.lock b/docs/static_site/src/Gemfile.lock
index 09dc815..6a4e2e9 100644
--- a/docs/static_site/src/Gemfile.lock
+++ b/docs/static_site/src/Gemfile.lock
@@ -37,7 +37,8 @@
       jekyll (>= 3.3, < 5.0)
     jekyll-watch (2.2.1)
       listen (~> 3.0)
-    kramdown (2.1.0)
+    kramdown (2.4.0)
+      rexml
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)
     liquid (4.0.3)
@@ -51,6 +52,7 @@
     rb-fsevent (0.10.3)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
+    rexml (3.2.5)
     rouge (3.17.0)
     safe_yaml (1.0.5)
     sassc (2.2.1)