Add :url option for cljdoc.edn sidebar links

[sritchie]

A note from @martinklepsch in Slack:

I would be open to a change to allow a :url key and showing that in the sidebar with adequate markings

This would allow forms like:

{:cljdoc.doc/tree
 [["Introduction" {:file "doc/intro.adoc"}]
  ["#sicmutils on Slack" {:url "https://clojurians.slack.com/blah"}]]}

[rollacaster]

If no one is working on this issue, I'd take it