docs(download page): Improve experience for people wanting to download "extra" modules

[petebacondarwin]

See #4811

[IgorMinar]

we should just say - use bower. no?

[petebacondarwin]

At the moment we have a "Download" button on the homepage, which has a nice UI for downloading the main angular.js file. But to download the other modules (such as angular-animate.js, etc) you have to click on the "extras" button, which simply takes you to the directory listing on code.angularjs.org. This is neither attractive or intuitive.

The simplest solution would be to ditch this "extras" link and replace it with a quick explanation that there are additional modules and link to the http://docs.angularjs.org/misc/downloading page.

[petebacondarwin]

This issue should be on the https://github.com/angular/angularjs.org project.