Implement cider-doc for keywords

[vemv]

Context

Compliment and Corfu can reach for cider-doc for describing a candidate.

When that candidate is a keyword, nothing is shown (because cider-doc isn't implemented for keywords), which depending on the Compliment setup, can result in this ugly buffer being shown:

Task

Implement, for now, a bare-bones code path for cider-doc such that we'll at least show the 'arities' that all keywords share ([map], [map not-found]).

If it's at hand, we can also show the spec, if found.

We can leave as a TODO showing the xrefs.

[vemv]

This isn't as important, following come Company commits (which will no longer show useless doc buffer)

(also, I had meant to open this under https://github.com/clojure-emacs/cider/issues - not cider-nrepl)