Consider adding Ember Data, Engines, and other top addons to "Ecosystem" on learn page

[Gaurav0]

I think this area is a great place for advertising how awesome the Ember ecosystem is, as long as we don't go overboard.

Top addons I would suggest:
ember-data
ember-engines
ember-cli-mirage
ember-concurrency

I would also suggest a link to Ember Observer at the end of this section.

Another alternative would be to only show what we have but add a "Additional Ecosystem page" of some kind.

Let me know if this needs a RFC.