Cleanup the assets before generating the guides

This will fix the issue that digest were being included into
digest paths.

guides/rails_guides/generator.rb

@@ -44,6 +44,7 @@ def generate
       if !dry_run?
         # First copy assets and add digests to make sure digest_paths are
         # present in generate_guides.
+        cleanup_assets
         process_scss
         copy_assets
         add_digests
@@ -128,6 +129,10 @@ def select_only(guides)
         end
       end
 
+      def cleanup_assets
+        FileUtils.rm_rf(Dir.glob("#{@output_dir}/{stylesheets,javascripts}"))
+      end
+
       def process_scss
         system "bundle exec dartsass \
           #{@guides_dir}/assets/stylesrc/style.scss:#{@output_dir}/stylesheets/style.css \