Add an info page

[phadej]

I was curious whether Hackage already accepts cabal-version: 3.12 (a patch was merged two months ago). Luckily there are candidates so I could just try, but it might be nicer if there were an actual page showing (at least some) information about the running instance.

[Bodigrim]

It's available from the footer on package pages, e. g., https://hackage.haskell.org/package/hashable says "Produced by hackage and Cabal 3.10.2.1".