Add terminology/references/dictionary

Add a page to explain Babel/js specific terms that users might not know at first

like: es6/es2015, TC39, plugin, preset, etc.

Not sure how we want to do this but at least we can come up with some terms, and then how to show it on the site and whether we should be linking to it in some way.

Start by reviewing the proposed terms in the issue and the existing Babel website documentation structure. Agree on the dictionary's scope, presentation, and linking approach before implementation. Done means the terminology page exists with useful explanations and an agreed path for users to find it.

Written by the indexing model from the issue text.