Overhaul Getting Started

[seancorfield]

We need clear guidance on CLI/deps.edn as well as Leiningen with some impartial discussion of both.

[seancorfield]

I've moved the old Getting Started to Getting Started with Leiningen and added a parallel Getting Started with Clojure CLI page. The Getting Started landing page now talks about why multiple tools exist and why you might choose one over the other. Or both.

I've also introduced the idea of "interactive development" via an editor connected to a REPL ("jack-in"), to distinguish that from just "working with a REPL" which was what this guide previously claimed as "interactive development".