Link core-packages page

[xtuc]

Following #1026. We are missing a link to it.

The page is already online http://babeljs.io/docs/core-packages/.

We could add it in the docs dropdown?

Or we could replace this page http://babeljs.io/docs/usage/api/ which has the same in it.

@hzoo @existentialism Any ideas?

[hzoo]

Ok do you want to just replace API with "Core Packages"?

[existentialism]

Not ideal, but until #1042 gets resolved, menu makes the most sense.

An interim solution, that we might want to consider, is grouping the items in the menu, with something like:

Setup
Editors
Caveats

-- Packages --
Core
Plugins
Polyfill

--- Usage --
.babelrc
CLI
babel-register

Bonus, we'll likely want to do similar groupings in the eventual side bar nav. Thoughts?

[hzoo]

Grouping sounds good