Update to latest document icons

[Jahnp]

If you've used OneDrive or a SharePoint Document Library recently, you'll notice that the file type icons have been refreshed with an updated look.

Take the Excel .xslx icon, for example. Here's the current icon (note that the visual artifact under the "X" does not show up at its intended size of 48px):

Here's the new icon:

These icons are actually available today via the @uifabric/file-type-icons package (the source for which is available in office-ui-fabric-react), which maps file extensions like .txt and .png to their appropriate PNG and SVG assets. However, these are neither available in Fabric Core, nor are they documented in the Brand Icons page of the Fabric website.

Fabric Core should be updated to include the new set of icons, its documentation page should be updated to include the full list of file types and sizes, and the docs should include good examples of how to use them.