Add JavaScript Guide to Intl article

[littledan]

MDN has lots of good reference documentation about Intl, but I can't find any articles which are a generic introduction to what Intl does in general, why you'd use it, and how it fits into a bigger program. I think it'd be helpful to add an entry to MDN's JavaScript Guide. It could be analogous to an article like Keyed Collections in the JavaScript Guide, or it could be in some more text in the Intl namespace article. I feel like the current resources can feel a bit low-level sometimes.

[romulocintra]

Agree @littledan , i think we should also help in some concepts that are not directly exposed into Intl API but are very important and the base for to the Intl work, concepts like CLDR's , Unicode , ICU... most of the people don't know about.