Site: Use kramdown for processing markdown

See discussion on https://groups.google.com/forum/#!topic/bazel-dev/puQn4173CXU

This is again https://github.com/bazelbuild/bazel/commit/ea4840042b0c087685ab4faffab58ba6906b06f5, but the server has been updated to a newer version of Jekyll. I've tried the Docker container locally and it worked well.

RELNOTES: None.
PiperOrigin-RevId: 221086654
diff --git a/site/_config.yml b/site/_config.yml
index 17986c7..b66b8da 100644
--- a/site/_config.yml
+++ b/site/_config.yml
@@ -1,9 +1,10 @@
 destination: production
-markdown: redcarpet
-redcarpet:
-  extensions: ["tables"]
+markdown: kramdown
+kramdown:
+  syntax_highlighter: rouge
+highlighter: rouge
+
 paginate: 10
-highlighter: pygments
 sass:
   sass_dir: _sass
 gems: [jekyll-paginate]