Make `cider-javadoc` work with Javadocs located within .jars

[vemv]

M-x cider-javadoc doesn't appear to work for .jar javadocs, as enabled by enrich-classpath.

I don't immediately know what the correct behavior should be tbh - perhaps uncompressing them to a tmp dir and then browser-opening that?

We also have the ~./cache/mx.cider/unzipped-jdk-sources dir. We could use a similar technique.