Update Data Viewer docs for 2.0

I don't think this covers all of CKAN's data preview capabilities anymore: http://docs.ckan.org/en/latest/data-viewer.html

It should explain what files CKAN can preview and what needs to be done to enable each kind of preview. What previews are available out of the box? Which require an extension like the recline-, json- and pdf-preview extensions or the datastore? Does a file need to be added into the filestore or datastore for the preview to work, etc?