Better document uv setup

I've been away most of today but will push a minimized one later. I still think local documentation on how to bootstrap a `uv` setup replicating what CI currently uses would be nice, for the next time someone wants to do this. (If that's even possible, given that it looks like my Python version might be part of the problem: somehow `uv` used 3.13 on my system but 3.12 on Phil's, despite his being on a later Ubuntu version.)

_Originally posted by @geekosaur in https://github.com/haskell/cabal/issues/11683#issuecomment-4173330337_