normal = just the base game = 6 sandbox locations to play and replay in different ways unlocking options and new start points, gear, endings etc.
silver = base + expansion 1 + couple of bits of gear
gold = base + expansion 1 + expansion 2 + more bits of gear
Here's some useless info from the publisher...
"Access to the upcoming Expansion Pass 1, which offers new Missions, a new Location, a new Sniper Map, new Challenges, new Weapons, and new Outfits."
"Access to the upcoming Expansion Pass 2, which offers new Missions, a new Location, a new Sniper Map, new Challenges, new Weapons, and new Outfits."
You can also get the legacy pack which adds all the content from Hitman 1... (although if you have that on Steam, I think you get that DLC for free)
Like you, I couldn't find anything which says what the expansion 1 + 2 actually are though. So I just bought the base game on cdkeys, with the intention to get the legacy pack when its suitably cheap or I played the base game too much.
I've played 4 missions so far and its technically very cool. I however find myself replaying Sniper Elite 4 though for its sneak about, drop-in and kill stuff nature which is more fun for me than Hitman which does not like you killing everyone in sight.