Browser quickstart v2

This updates the browser quickstart and follows the suggestions by @ericf  in terms of organization and content. 

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I think we need to step back and figure out what the goals are for getting people to use Intl in the browser.

*  I would start with examples around Number and Date formatting (which are built into the modern browsers). [done]

* From there we can talk about what browsers support window.Intl, and then how to conditionally load the Intl.js polyfill in the browsers which do not support it. [done]

* Now that the basics of using Intl are established we could then move on to more complex messages, and introduce intl-messageformat, give some examples, touch on the feature highlights, link off to its main docs page, etc. This would also require showing people how to load this onto their page. [done] 

* Finally, I think we'd then provide people with a menu on where to go next, and that would be links to each of the template engine pages. [There are links at the bottom of the article to the future locations, and we can add the menu once that aspect of the site has been designed]

"Browser Compatibility" is on the same level as "Date Formatting" and "Number Formatting". The headings look right from here.