Design Request: Highlight Advanced Topics in the Guides

Part of #259

We need a design idea for advanced topics in [the Ember Guides](https://guides.emberjs.com/release/)!

We believe that some of the topics in the Guides are more suitable for beginners while others are better suited for experienced users. We'd like to highlight advanced topics in a more distinct and visual way. Some ideas we had so far, was highlighting the respective section in a different color or adding a distinct icon to the sidebar where the advanced topic is mentioned and / or to the title tag of the advanced section.

For more context, please take a look a look at the [related Quest Issue](https://github.com/ember-learn/guides-app/issues/259).

If anyone would like to pick this up, please comment below.