Sadly, none of the Pixel phones support HDMI or DisplayPort. The only way around that is to use the DisplayLink Presenter app with a DisplayLink compatible dock or dongle. DisplayLink creates a video stream that the dock/dongle can decode. The downsides are it can't display protected video content (such as Netflix), its a mirror of your screen (no desktop mode), and it can get a little choppy at times.
Google intends for the entire Pixel line to use Chromecast if you want to display your phone on another screen.