These are the docs for Metabase v0.58. Check out the docs for the current stable version, Metabase v0.63.
Working with Clojure
Check out Clojure for the Brave and True. It’s free online.
If you don’t feel like reading a whole book, Mark Volkmann’s Clojure tutorial is another good starting point.
Read docs for other versions of Metabase.