The alignment has drifted such that when my front wheel is looking straight ahead, my handlebar is not perpendicular to the wheel.
How do I fix this at home?
Nowadays most bikes have a 'threadless' headset. The steerer tube of the fork protrudes out of the frame's head tube and the stem clamps onto it. The stem serves to hold the whole system together and apply a little pre-load force onto the bearings to remove any play or movement. Without this pre-load the bearings will become damaged as the fork rocks in the frame slightly as you go over bumps.
If you have a threadless headset and your stem has rotated on the steerer tube, the chances are the clamp bolts are too loose and you have lost the bearing pre-load.
It's not too difficult to re-adjust the stem (and re-align the stem and wheel at the same time). There are many video's on YouTube that show you how to do it. Here's a reasonably good one.
If you don't feel happy doing this adjustment a local bicycle repair shop will do it in under 10 minutes.
If you have an old style threaded headset, the stem is simply secured by a bolt in the top of it. Here is a good tutorial on how to adjust it. The process is fairly fool-proof.
The bike in your link appears to have an "old fashioned" threaded fork.
With the appropriate sized Allen wrench, loosen the bolt through the top of the stem, maybe 2 turns. Then see if you can twist the stem. If it's still stiff unscrew the bolt another turn or two (so that the head is "proud" of the stem), then give the bolt a modest whack with a hammer, to knock the "wedge" inside the stem loose. (When you do this you'll see the bolt drop down a bit.)