Add tabindex for fallback code view

Author: timblair

Diff for: lib/govuk_tech_docs/tech_docs_html_renderer.rb

@@ -123,7 +123,7 @@ def block_diagram(code)
         if ok && File.exist?(output_path)
           File.read(output_path)
         else
-          "<pre>#{code}</pre>"
+          %Q{<pre tabindex="0">#{code}</pre>}
         end
       end
     end