Their actual website https://godsunchained.com/ is pretty darn good. There's a "Learn" tab there that has access to some articles and tutorials. You're honestly best off just starting by downloading the game client and jumping in. They have a built in gameplay tutorial that's just a few minutes long with the basic game mechanics and then awards you a few packs of cards on completion. There's also a bunch of prebuilt starter decks to play around with that are actually fairly competitive right out of the box once you learn the basics.

You'll need or want to have an Ethereum wallet if you don't already, although I don't think you're required to link it right away... though I could be wrong about that.

I'm not familiar with Legends of Runeterra, but if you have any experience with collectible card games then Gods Unchained will be fairly easy to pick up as an iteration on that genre.