Changing the way docset home is determined

Previously one topic set built at the root. It's index became the docset
index. Now all topic sets build in dir named for topic-set-id and one is
designated home-toopic-set in config. Ability to specify a different dir
name of topic set removed. First step of breadcrumb now link to home
index page, but this should actually link to docset toc, which is not
generated yet. Then there is the question of how to get back to docset
home page. Separate link?