docs: Adding screenshot for Now Playing page

A follow-up to Chris Cartland's changes to the Now Playing
page (http://ag/831240 ), adding a supporting screenshot
that shows a Now Playing card with media in the background.

New PNG added at:
docs/html/images/training/tv/playback/now-playing-screen.png

Updated to use new version of PNG with more whitespace cropped
out (thanks Andrew!)

Bug: 26345930
Change-Id: I217fcc062f80734aebe32ec3a94ec7595756e78c