Which side of the oven tray is the front one? Why?

It doesn't really matter most of the time, but the sloped side should be the one facing you. Any rack designed to go inside the tray would better align with any grill (broiler) elements in that position. It also makes access easier if you don't have any rack in it & need to reach into the tray itself.

The holes may be for some additional attachment, or simply where the tray was hung whilst it was being enamelled. Any kind of heat-set powder coating needs to have a hanger which won't interfere too much with the eventual finish. Often they're placed out of sight, but that's not always possible, depending on the overall design.
Sometimes if you look, or even feel carefully, you can identify where the hanging wire/hook went, by the irregularity in the surface at that point.

Two lovely old enamelled trays showing the holes, with a cheap teflon one underneath, no holes

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You can cook pizza directly on the tray. With the slope at the front, it's possible to put it in and take it out with a pizza peel. The other way round it's much harder. This way you can preheat the tray and don't have to take it out to load your pizza onto it

I prefer a stone, but this worked well when I was between pizza stones

I believe that the slanted side goes on the front of the oven. The design makes it easier to slip one's over mitten underneath the tray.