Implement a Tab component that can be used across the different websites

As part of the discussion on https://github.com/ember-learn/ember-website/pull/530 we identified that we should probably move the tab implementation into the styleguide.

I think that is a good idea but we should implement the styleguide tabs component in collaboration with someone working on another Ember website (such as the api docs) so that the API can be discussed and finalised in such a way that is flexible enough for all websites and makes sure the designs are cohesive 👍

Start by reading the discussion in ember-website#530 and reviewing how tabs are implemented in the Ember website and API docs. Coordinate with another Ember website contributor to agree on a flexible API and cohesive design; done means a shared styleguide tabs component works across the participating websites.

Written by the indexing model from the issue text.