Leftovers from #2585

[alitaheri]

#2585 had some work undone. But since the work was huge I preferred to postpone these works:

• Replace Style mixin with Util functions. -> Migrate components to ES6 Classes #458
• Add info on this in the documentation.
• Doc pages should be moved to Page.jsx instead of index.jsx see here. [Docs] Migrate to the new naming strategy #2628
• Address deprecation warnings in the doc pages Postponed for next release
• Fix AutoComplete, it should use Popover.