Add details on deps.cljs

The deps.cljs file is discussed by example in a couple of places:

• https://clojurescript.org/reference/dependencies
• https://clojurescript.org/reference/packaging-foreign-deps
• https://cljs.github.io/news/0.0-2719

I'd really like to see a slightly more formal description of how this file works. Questions:

• Is the deps.cljs filename hardcoded?
• Does it only work inside jars? (Only at the top level directory?)
• Can there be more than one in a project? How do they interact if so?
• Can it contain anything other than a :foreign-libs key?

(I think part of my confusion lies from the fact that I'm a bit fuzzy on the mechanics of what happens when you add a dependency string to a lein project file. I always thought that the dependency strings only affected the downloading of dependencies to disk--not compiler options. I think the answer is that the dependency vector downloads the jars to disk and the presence of a deps.cljs file on disk affects the compiler, but it'd be great to have this spelled out.)

CC: @johnmn3

Start by comparing the existing dependency and packaging documentation at the three linked URLs, then verify how deps.cljs relates to a Leiningen dependency declaration. Document the filename, supported locations and keys, handling of multiple files, and the download-versus-compiler behavior, with examples that answer each question.