Add a CSS codelab (#2646)

* feat: Initial commit of CSS codelab solution

* fix: Remove classic.css

* feat: Add CSS codelab Markdown

* feat: Add CSS codelab screenshots

* chore: Fix categories-logic.png

* chore: Clean up CSS codelab Markdown

* fix: Run prettier on CSS file

* fix: Exempt codelab solution from eslint

* fix: Add prettier fixes to doc

* fix: Integrate Rachel's comments

Author: rpbourret