It'll show up in Minecraft Dungeons 2. The dimension itself won't come out in Minecraft until next year.

I feel the advancement tab is decent for finding stuff without leaving the game. It's still looking up hints, just ingame. I'd love for them to add readable books in ancient ruins to give you hints instead. Idk how they'll give hints for this tho

No way to tell until the update drops I imagine, but the nether and the wither can be organically encountered/spawned (ruined portals and a specific painting) so I think there's a pretty good chance