Website and Documentation Doesn't Support Rouge Syntax Highlighter

When writing CASSANDRA-14827, I noticed that the Rouge syntax highlighter used by Jekyll is not configured properly. The CSS used by the highlighter is not available and there are some issues with line-wrapping when using code block syntax (```~~~ syntax). The source markdown in CASSANDRA-14827 uses <div><div><pre> around code blocks instead. This works around the line-wrap issues but not the Rouge CSS issues. 

When adding this, we should take care to not break other parts of the site that have hand rolled highlighting/formatting (like the CQL docs).