Let {module}/doc/ list all known versions and their corresponding docs

[simonbasle]

There is currently no way to see pre-formed links to older versions of the docs (reference, javadoc, kotlin docs...).

It could be useful to have /docs/{module}/ pattern pull up a list of all known versions for that module, as links to the javadoc. Maybe even list refdocs and kdocs when relevant? (see #43 for a potential caveat: don't generate kdocs for some versions)